Ferrite-martensite Dual Phase Steel Research Articles

Achieving high strength and low yield ratio by constructing the network martensite-ferrite heterogeneous in low carbon steels

In this research, focusing on low-carbon steel, a martensite-ferrite heterogeneous structure dual-phase (MFDP) steel with a network morphology where ferrite is surrounded by martensite was obtained via cyclic annealing and subcritical quenching heat treatment processes. With the initial microstructure of ferrite and lamellar pearlite, a spherical pearlite and martensitic structure surrounding the ferrite was first obtained by applying the cyclic annealing process near the Ac1 temperature. Subsequently, the annealed structure was subjected to subcritical quenching heat treatment, thereby establishing a network-like martensite-ferrite dual-phase heterogeneous structure and named N-760 °C and N-780 °C. In comparison with the ferrite-martensite dual-phase steel where ferrite envelopes martensite, N-780 °C witnessed a marked increase in tensile strength and uniform elongation, while the yield ratio dropped by 20 %. Through cyclic loading and unloading tensile tests, it was found that the N-760 °C showed a more obvious heterogeneous deformation-induced (HDI) strengthening effect. The results from electron backscattering and transmission electron microscopy indicate that, in the N-760 °C, a small quantity of dislocations is produced in the ferrite due to the martensitic phase transformation prior to the tensile test. During the tensile process, as the strain increases, the ferrite undergoes significant deformation, and the intragranular dislocations re-arrange to form dislocation cells and deformation-induced grain boundaries (SIBs). Meanwhile, geometrically necessary dislocations (GNDs) accumulate at the ferrite/martensite interface. Therefore, the non-coordinated deformation between the mesh-like dual-phase microstructure offers additional HDI strengthening for MFDP steel.

Quantitative characterization of local deformation-fracture behavior in ferrite-martensite dual-phase steels with different martensite distributions

Low-carbon dual-phase (DP) steels, composed of a soft ferrite phase and a hard martensite phase, are known as promising advanced high-strength steels (AHSSs) due to their high strength and good ductility at low fabrication cost. However, the deformation behavior of DP steels is still not fully understood because of their complex mixed-phase distribution and mechanical interactions between two phases during deformation. The present study quantitatively investigated the effect of martensite distribution on mechanical properties and local deformation-fracture behavior, using the digital image correlation (DIC) technique. Two types of DP structures were prepared: one with a chained martensite distribution (chained DP) and one with an isolated martensite distribution (isolated DP). The chained DP specimen exhibited a superior tensile property, achieving both high strength and large ductility compared to the isolated DP specimen. DIC strain analysis revealed that the chained DP structure showed relatively homogeneous deformation due to the greater contribution of martensite to plastic deformation. In contrast, the isolated DP specimen experienced significant strain localization in the soft ferrite grains. Despite high global strains, non-plastically deformed zones were observed in the central regions of large, isolated martensite particles. Notable differences in micro-void evolution were also observed between the two specimens. The chained DP specimen had a large number of randomly distributed micro-voids, ranging from 0.5 μm to 2 μm in size. In contrast, the isolated DP specimen contained fewer micro-voids, with some aligned at an angle of ∼65° to the tensile direction, potentially leading to early tensile fracture.

Semi in-situ investigations on deformation-induced micro-damage in high-strength dual-phase steels

This work investigates deformation-induced micro-damage modes in ferrite-martensite dual-phase (DP) steels utilizing a state-of-the-art semi in-situ deformation apparatus integrated with scanning electron microscope. By integrating secondary electron imaging, electron backscatter diffraction, and electron channeling contrast, we provide comprehensive statistical results on micro-damages and strain localization events of their formation in DP780 and DP980 steels. The current observations indicate that interface decohesion plays a critical role in DP780, particularly under plastic instability. Based on maps of Kernel averaged misorientation, interface decohesion arises from the accumulation of geometrically necessary dislocations, which serve to compensate for the deformation incompatibility between ferrite and martensite. In contrast, the dominant damage mode in DP980 under plastic instability involves ferrite cracking within martensite-surrounded ferritic grains. This type of micro-damage is a result of strain localization by shear bands that accommodate the global deformation while the grain is constrained by martensite islands. Martensite cracking does not show as predominate sites under the condition of plastic instability in both steels. In summary, our findings highlight the importance of deformation incompatibility and constraint on occurrence of micro-damages through distinct strain localization events, which are determined by both microstructure of damage site and its environmental microstructure. In this work, we shed light on the role of microstructural engineering as a critical strategy for enhancing the resistance of DP steels or hetero-phase steels against deformation-induced damage.

Microstructure, mechanical properties and strengthening mechanism of high strength hot-rolled ferrite-martensite dual phase steel

This study is focused on investigating the microstructure and mechanical properties of high strength ferrite-martensite (F-M) dual phase steels manufactured through a simple hot rolling and step cooling. Additionally, it seeks to elucidate the strengthening mechanism of the ‘unexpected microstructures’, namely lower bainite and auto-tempered martensite, within F-M dual phase steels. During the step cooling process, the morphology of ferrite is determined by the air-cooling temperature, while the ferrite content is affected by the air-cooling temperature and time. Since the morphology of continuously cooled bainite is determined by the cooling rate, granular bainite is formed in the air-cooling stage and lower bainite is formed in the laminar water-cooling stage. Combined with the reasonable microstructure construction and the strengthening effect of ‘unexpected microstructures’, the sample air-cooled at 700 °C for 10s achieves the best comprehensive mechanical properties (yield strength: 1155 MPa, tensile strength: 1511 MPa, total elongation: 11.5%, impact toughness in −20 °C: 16.6 J). The strengthening effect of ‘unexpected microstructure’ is mainly to improve strain strengthening and impact toughness. The presence of lower bainite and auto-tempered martensite both promotes the incompatible strain during the deformation, resulting in higher strain hardening. This effect is further enhanced by the segmentation of the prior austenite grains by lower bainite and the plastic constraints imposed by surrounding martensite during deformation. Auto-tempering contributes to an increase in impact toughness by reducing areas with high dislocation density. The improvement of impact toughness by lower bainite is mainly attributed to the increase of high angle grain boundaries when partitioning the prior austenite.

Effects of the thermomechanical treatment on the microstructure and mechanical properties of ferrite–martensite dual phase steel

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Novel miniature in situ hole expansion test coupled with microscopic digital image correlation.

The hole expansion ratio (HER) test is used to determine the stretch-flangeability of materials. Standard HER tests are performed on specimens sized a few tens of centimeters, termed macro-HER tests. This leads to significant material wastage due to the destructive nature of the tests. No information at the microstructure length scale is obtained, and the results suffer from user uncertainty in the identification of the through-thickness crack. This paper presents a novel miniature HER setup (termed micro-HER test), in which miniature specimens are tested inside a scanning electron microscope (SEM). The deformation is imaged from the top using the secondary electron detector installed in the SEM, coupled with the digital image correlation (DIC) technique, allowing for measurement of full field strains at the microstructural scale and identifying their deformation/fracture mechanisms. As a case study, six different steel grades were tested to measure their micro-HER values and compare them with the corresponding macro-HER values. The latter were found to be higher for the more ductile grades of steel. Late detection of through-thickness cracks and thicker samples leading to a higher volume of plastic deformation could contribute to this overestimation of values in macro-HER tests. DIC results from micro-HER tests on a ferrite-martensite (10% volume fraction) dual-phase steel showed high magnitudes of strain localization at the ferrite-martensite interfacial regions, indicating that such interfaces might be hotspots for failure under triaxial stress states. The challenges and errors associated with the measurements are also discussed.

Bending deformation behavior of a 1000 MPa grade dual-phase steel with superior bending toughness

Poor bending toughness is still a critical drawback of high-strength ferrite-martensite (F-M) dual-phase (DP) steel, which is unfavorable to their collision safety. In this study, a 1000 MPa grade F-M DP steel with excellent bending toughness was fabricated by reducing the strength difference between ferrite and martensite via introducing solid solution strengthening (Si), precipitation strengthening (VC) and reasonable tailoring high martensite content. The obtained DP steel exhibits a high ultimate tensile strength (UTS) of 1123 MPa, together with a good total elongation (TE) of 16.0 % and an excellent maximum bending angle (αf) of 98°. The αf of the obtained DP steel is obviously higher than that of 62–80° for reported DP steels with similar UTS levels. The related bending deformation and fracture behavior are emphasized and indicate that high bending toughness in obtained DP steel can be attributed to the excellent coordinate deformation between ferrite and martensite, improved martensite deformability and crack blocking by the fibrous deformed martensite.

Understanding fusion zone hardness in resistance spot welds for advanced high strength steels: Strengthening mechanisms and data-driven modeling

The fusion zone (FZ) hardness of resistance spot welds is a crucial factor affecting the performance and durability of the welds. Failure mode transition, tendency to fail in interfacial mode, interfacial failure load, and the impact of liquid metal embrittlement cracks on the weld strength are influenced by the FZ hardness. Therefore, accurately predicting the FZ hardness in resistance spot welds made on automotive steels is essential. A simple thermal model is used to calculate the time required for the temperature to drop from 800 °C to 500 °C (Δt85). With the aid of continuous cooling transformation (CCT) diagrams and experimental confirmation, it is shown that in most automotive steels, the FZ exhibits an almost full martensitic microstructure. Generally, the FZ hardness tends to increase in the order of interstitial-free (IF), drawing quality specially killed (DQSK), high strength low alloy (HSLA), ferrite-martensite dual-phase steels, transformation induced plasticity (TRIP), and quench & partitioning (Q&P) steels. To predict the FZ hardness, data-driven regression-based models have been developed based on carbon content, carbon equivalent concept, and the strengthening mechanisms of the martensite. Among these models, the model based on martensite strengthening mechanisms showed the best performance and robustness in estimating the FZ hardness. The assessment of the strengthening mechanism of the FZ showed that most important factors determining the FZ hardness are dislocation hardening due to carbon atoms, interface hardening due to block boundaries, and solid solution hardening due to substitutional alloying elements, respectively. A simple relation is proposed to estimate the load-bearing capacity of automotive steel spot welds during interfacial failure based on the FZ hardness.

Embrittlement mechanism of ferrite-martensite dual-phase steel during strain-baking

Press forming and paint curing are necessary in automobile production. However, strain and baking effects induced by these processes cannot be ignored in steel plates. Previously researchers have focused on the baking hardening effect but neglected the embrittlement issue. Baking embrittlement occurs when the elongation of ferrite-martensite dual-phase (DP) steel decreases significantly after strain-baking. In this work, the baking embrittlement value (ΔE) was defined for the first time as an evaluation index, and the baking embrittlement mechanism of DP steels was clarified. The baking embrittlement process of DP steels was divided into two stages. The first stage consisted of dislocations pinned by C clusters or low-temperature carbide, while the second stage was the tempering effect of martensite. Furthermore, the volume contraction of martensite led to the release of internal stress, resulting in an increase in the critical stress for dislocation movement. Both processes reduced dislocation mobility. In addition, by calculating the geometrically necessary dislocation (GND) densities before and after strain-baking, we found that the multiplication ability of dislocations was weakened.

Microvoid Formation of Ferrite-martensite Dual-phase Steel via Tensile Deformation after Severe Plastic Shear-deformation

Microvoid Formation of Ferrite-martensite Dual-phase Steel via Tensile Deformation after Severe Plastic Shear-deformation

Microvoid Formation of Ferrite-martensite Dual-phase Steel via Tensile Deformation after Severe Plastic Shear-deformation

One of the objectives for the development of high-strength dual-phase (DP) steel is improving the stretch-flangeability. Large-strained sheared edges are deformed and frequently cracked during stretch-flange formation. Considering shearing as the first deformation, the stretch-flange deformation may be regarded as a secondary deformation. To improve the stretch-flangeability of the DP steels, many researchers have analyzed the microvoid formation. However, in these analyses, the shearing process was not considered. With this background, ex-situ mini-bending tests combined with scanning electron microscopy (SEM) monitoring of microvoid formation were conducted during the secondary deformation. Prior to the secondary deformation, several microvoids were observed on the sheared surface and fine subgrains formed in the ferrite. During secondary deformation, the preliminary microvoids present at the ferrite-martensite interface propagated into the ferrite phase. In contrast, this behavior was not observed for the reamed surface deformation, which was formed without preliminary deformation. Furthermore, microvoids were initiated on ferrite grains that were not present at the ferrite-martensite interface, and martensite islands were not cracked during secondary deformation. This result is noteworthy because martensite cracking was the main factor involved in microvoid initiation, in the absence of shearing. Electron backscattering diffraction analysis revealed that the work hardening of ferrite, prior to the secondary deformation, caused a deviation in the strain concentration sites from those found in the reamed surface deformation. Therefore, this study elucidated microvoid formation on preliminary deformed surfaces via shearing and provided insights for material development considering deformations on the sheared surfaces of materials.

Abnormal Trend of Ferrite Hardening in a Medium-Si Ferrite-Martensite Dual Phase Steel

In this paper, the effects of carbon, Si, Cr and Mn partitioning on ferrite hardening were studied in detail using a medium Si low alloy grade of 35CHGSA steel under ferrite-martensite/ferrite-pearlite dual-phase (DP) condition. The experimental results illustrated that an abnormal trend of ferrite hardening had occurred with the progress of ferrite formation. At first, the ferrite microhardness decreased with increasing volume fraction of ferrite, thereby reaching the minimum value for a moderate ferrite formation, and then it surprisingly increased with subsequent increase in ferrite volume fraction. Beside a considerable influence of martensitic phase transformation induced residual compressive stresses within ferrite, these results were further rationalized in respect of the extent of carbon, Si, Cr and Mn partitioning between ferrite and prior austenite (martensite) microphases leading to the solid solution hardening effects of these elements on ferrite.

Corrosion mechanism of advanced high strength dual-phase steels by electrochemical noise analysis in chloride solutions

The corrosion behavior of ferrite-martensite and ferrite-bainite dual-phase steels in NaCl, CaCl2, and MgCl2 solutions was studied by cyclic potentiodynamic polarization (CPP) and electrochemical noise (EN) techniques. The results showed a trend for mixed corrosion due to no passivation by a protective oxide layer. For the ferrite-martensite dual-phase steels, the ferrite phase was preferentially corroded compared with the martensite phase in test solutions. Bainite-microstructure in ferrite-bainite dual-phase steels was preferentially corroded compared with ferrite-phase in CaCl2 and MgCl2 solutions. The corrosion behavior in test solutions in dual-phase steels could be divided into three steps: dissolution in anodic-phase/microstructure, non-uniform corrosion, and localized corrosion. The results indicated that factors such as galvanic coupling between ferrite/martensite, ferrite/bainite, metallic-matrix/inclusion-phases, metallic-matrix/secondary phases, and self-corrosion of each phase and microstructure contributed to the non-uniform and localized corrosion in dual-phase steels.

Qualities of electron backscatter diffraction patterns and image contrast from a ferritic-martensitic steel microstructure

Quality metrics of electron backscatter diffraction patterns were constituted with a few descriptors of the characteristic intensity distribution across diffraction bands. The image contrast by these metrics was compared quantitatively with a ferritic-martensitic dual phase steel. The metrics which described the sharpness of bands were superior to those that represented the contrast from a background intensity. In addition, the ones that were defined around the edges of a band presented better contrast than those in the interior. Pattern degradation by martensite was approximated with lattice rotations in an interaction volume, and the sensitivity of the metrics to pattern degradation accords well with their behavior on the experimental patterns. The metrics defined around the band edges were more prone to instability in the evaluation processes. A new metric which described the sharpness in the interior of bands presented a compromise between the sensitivity and the stability. • Pattern quality metrics of EBSD were compared with a ferritic-martensitic steel. • Metrics by band sharpness presented higher-contrast micrographs than by contrast. • The edges of a band gave better contrast but poorer stability than the interior. • A metric of sharpness in a band interior presented an excellent compromise.

Effects of spot welded parameters on fatigue behavior of ferrite-martensite dual-phase steel and hybrid joints

In this paper, the fatigue behavior of resistance spot welding in the ferrite-martensite dual-phase steel joints with one and three spot-welds and hybrid joints have been investigated. Experimental parameters are welding time, electric current’s intensity, and the compressive force applied. Design of experiments were done by Taguchi statistical method and because of the number of studied parameters and factors. In order to predict the optimal parameters of the resistance spot welding, signal to noise ratio is used in Taguchi method. Also, scanning electron microscope (SEM) images of the fracture surfaces of specimens are investigated to evaluate the effect of studied parameters from a morphological viewpoint. The results show that the effect of electric current on the fatigue life of joints is more than other parameters but its effectiveness diminishes as it increases. The adhesive on the hybrid joints also has a significant effect on the increase of its fatigue life and fatigue strength.

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.

Crack Initiation and Propagation in Dual-phase Steels Through Crystal Plasticity and Cohesive Zone Frameworks

Ferrite-martensite dual-phase (DP) steels have become popular in weight-reduced automotive components due to their straightforward thermomechanical processing and excellent mechanical properties. Even though it has been a popular and productive area of research, there are still many open questions related to their interesting microstructure. Being composed of brittle martensitic clusters dispersed in a ductile ferrite matrix, DP steels benefit from the properties of both phases, which induce interesting failure mechanisms at the same time. The microstructural parameters such as martensite volume fraction, grain size, carbon content, martensite/ferrite morphology, ferrite grain size, and texture, as well as micro and mesoscale segregation, make them difficult to analyze. However these aspects have to be studied in order to link the underlying microstructural influence to macroscopic behaviour. Therefore, the modelling techniques at the microstructure scale should be utilized for physical understanding. In this work, a multi scale modeling strategy is followed for the analysis of plastic deformation and failure in DP steels. A rate-dependent crystal plasticity framework and the isotropic J2 plasticity model are employed for the simulation of the plastic deformation in ductile ferrite phase and in the hard martensite phase respectively at the RVE scale. Moreover, the intergranular cracking is studied by the incorporation of a cohesive zone model at the ferrite-martensite grain boundaries and the failure in martensite phase is addressed through a ductile failure model. The effect of microstructural parameters on both plastic and fracture behaviour is analyzed under axial loading conditions.

The effect of cold-rolling prior to the inter-critical heat treatment on microstructure and mechanical properties of 4340 steel with ferrite – Martensite microstructure

This study investigates the effect of cold rolling prior to inter-critical annealing on microstructure and mechanical properties of ferrite-martensite dual phase steel. Samples were heated to 850 °C for 1 h followed by oil quenching, then the steel sheet were cold rolled by 0%,10%,15% and 20% reduction in thickness. The inter-critical annealing treatment (750 °C, 120min) was performed to generate a ferrite-martensite microstructure. Microstructural studies showed that increasing the applied cold rolling, leading to increase in volume fraction of martensite and decrease in ferrite grain size. Mechanical properties of dual phase steel were measured by tensile, impact and hardness tests. Results showed that ultimate tensile strength, yield strength, micro-hardness and toughness (e.g. total elongation, uniform elongation and impact energy) increased with increasing the applied cold rolling. Improvement of mechanical properties were related to increase in martensite volume fraction and ferrite grain refinement. Analysis of strain hardening behavior of DP steels, by Hollomon analysis, showed two stages of strain hardening corresponding to ferrite deformation and co-deformation of ferrite and martensite, respectively. The strain hardening exponent of first stage (nI) increased with increasing volume fraction of martensite. The energy absorption capacity (UE × UTS) increased with increasing cold rolling deformation.

The Role of Phase Hardness Differential on the Non-uniform Elongation of a Ferrite-Martensite Dual Phase Steel

The Role of Phase Hardness Differential on the Non-uniform Elongation of a Ferrite-Martensite Dual Phase Steel

Effect of the Initial Microstructure and Thermal Path on the Final Microstructure and Bendability of a High Strength Ferrite-martensite Dual Phase Steel

Effect of the Initial Microstructure and Thermal Path on the Final Microstructure and Bendability of a High Strength Ferrite-martensite Dual Phase Steel