Dual Phase Steel Grades Research Articles

Martensite size and morphology influence on strain distribution and micro-damage evolution in dual-phase steels; comparing segregation-neutralised and banded grades

A change in strain partitioning and microscale failure mechanisms in dual-phase (DP) steel was found when both the morphology and distribution of martensite were altered compared to a banded DP steel grade benchmarked against a specific commercial DP grade. To achieve a DP microstructure with equiaxed and well-dispersed martensite, the concept of segregation neutralisation was utilised, where the ratio of Mn to Si elements was decreased (from 7.4 to 0.3) to neutralise the effect of Mn segregation on generating the banded martensite. A combination of micromechanical modelling simulations and in-situ notch tensile testing (within an SEM) was employed to compare the micro strain field and void formation rate between the segregation-neutralised and the benchmark grades. The benchmark grade showed extensive void coalescence along the direction of shear bands in the tensile sample after the average tensile strain of 30%. In contrast, no void coalescence was observed in the segregation-neutralised DP steel even at the average tensile strain of 80% just before failure. As a result, post-uniform elongation in the segregation-neutralised grade increased to 63.3%, compared to 30.1% in the benchmark grade.

Effects of spatial microstructure characteristics on mechanical properties of dual phase steel by inverse analysis and machine learning approach

This work aims to investigate complex relationship between microstructure characteristics and mechanical properties of dual phase (DP) steel through an inverse analysis based on Markov chain Monte Carlo (MCMC) method combined with meso-scale material modelling. In this framework, a machine learning approach as surrogate model was developed, in which support vector regression (SVR) and artificial neural network (ANN) were trained using results from representative volume element (RVE) simulations coupled with damage model, which were previously calibrated with experimental data of commercial DP steel grades. Moreover, specific microstructure descriptors including Moran’s index, martensite band index and martensite orientation were proposed for representing effects of spatial distributions of martensitic phase. As a result, inverse predictions of microstructure characteristics of DP steels for achieving defined yield strength, tensile strength, uniform elongation and toughness were presented. The inverse analysis could solve the non-uniqueness of structure–property relationships of steel, whereby significances of dispersed structures and aligned martensite bands were highlighted in details. The approach fairly dealt with multi-target optimization and high dimensional problem, which can be further applied as a guideline for designing DP microstructures with enhanced mechanical properties.

Understanding the damage initiation and growth mechanisms of two DP800 dual phase grades

Dual phase (DP) steels are amongst the most widely used structural steels for automotive applications. It is essential to understand the damage initiation and damage growth in these high strength steels and further shed light on improving mechanical properties. In this work, two DP800 dual phase grades are investigated, which exhibit identical ultimate tensile stress but significantly different elongation in the uniaxial tensile test. To explain the difference in ductility, particularly described by uniform elongation, we investigate the damage initiation and growth mechanisms by analyzing microstructural changes upon deformation, such as voids, dislocation structures and the grain morphology. Furthermore, ferrite micropillars in pre-strained samples are tested in situ to capture the strain hardening capability of ferrite. We found that the DP steel with harder martensite and softer ferrite exhibits more damage initiation sites after deforming to an identical strain. However, void growth is much slower compared to the DP steel grade with fewer initiation sites. We explain the suppressed void growth by significant strain-hardening of ferrite surrounding the voids, which is observed in the micropillar compression experiments. The improved strain hardening of ferrite originates primarily from the difference in chromium content considering the negligible influence of dispersed particles.

Effect of strain rates on the forming properties of DP Steel at high-temperature

Dual Phase steel grade, with its remarkable combination of features including formability and resistance to high temperatures, has been a revolutionary alloy in modern industry. It is a useful material that may be applied to many different tasks. It presents us with the task of examining its characteristics in response to changes in critical parameters such as temperature and loading rate during stretching operations. Understanding the formability behavior of DP steel sheets, which have a thickness of 1 mm, at various increased temperatures and strain rates was the main goal of the investigation. We investigated and examined the tensile characteristics at 650 and 750 degrees Celsius in the first investigation. On the other hand, because of flow stresses in the material at higher temperatures, elongation has decreased in value as deformation rates have increased. In addition, we plotted the FLDs experimentally and examined the formation behavior while performing the Nakazima Test on specimens with a thickness of 1 mm. Furthermore, a limiting dome height (LDH) study was conducted on a laboratory scale. In terms of temperature, the LDH was shown to be higher at 750 and at 0.001/s

Towards ultra-high strength dual-phase steel with excellent damage tolerance: The effect of martensite volume fraction

Ultra-high strength (> 1 GPa) dual-phase (DP) steels have been extensively investigated in literature. Yet, the damage tolerance of these DP steels remains mostly unexplored, whereas this parameter is crucial for the sheet formability and anti-crushing performance. In this work, a medium-Mn DP steel was proposed in order to develop strong but damage-resistant DP steel by refining the microstructure. Ultra-fine grained (< 1 µm) DP steels were obtained by a simple cold rolling and intercritical annealing process. The developed DP steel can achieve an outstanding ultimate tensile strength (UTS) of over 1470 MPa with appreciable elongation to failure (EtF) of over 9 % when the martensite volume fraction (Vm) is 55 %. Higher Vm results in higher strength. However, the fracture strain, which is connected with damage tolerance, decreases significantly with increasing Vm, and it is even much lower than that of a full-martensite steel. Furthermore, the necking behaviour transfers from diffuse plus localized necking to diffuse necking with increasing Vm. The sufficient ferrite content in the proposed DP1470 steel enables void growth after nucleation, consequently leading to the occurrence of localized necking and higher fracture strain. Thus, a moderate Vm ranging from 45 % to 65 % is suggested for the development of a 1470 MPa grade DP steel with excellent damage tolerance.

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.

EFFECT OF RETAINED AUSTENITE ON PLASTIC CHARACTERISTICS OF DUAL PHASE STEELS

In terms of the current trend of research and development of new materials and optimization of current materials in the automotive industry, the greatest attention is paid to progressive high-strength dual-phase (DP) steels with increased stampability, which are designed for cold stamping for specific internal car body components of the current market. New grades of DP steels provide a combination of high strength and good formability and contribute to the weight savings of vehicle parts by 10 to 20 %, compared to current DP grades. Thanks to their top properties, DP steels with increased formability can absorb more crash energy using less steel. As a result, high-strength DP780GI and DP780GI-HF materials of first generation (hereinfater DP780GI-HF) were analyzed. The stampability improvement of DP steels was demonstrated by the experimentally determined Forming Limit Curves for both steels.

Mechanical Behavior of Multi-Phase Steels Comprising Retained Austenite.

The retained austenite (RA) in advanced high-strength steel (AHSS) grades, such as dual-phase (DP) steels, plays an important role on their formability. Thanks to the transformation-induced plasticity (TRIP) effect that occurs during the mechanically induced transformation of RA into martensite, additional ductility is obtained. Martensite has a higher flow stress than austenite; hence, the transformation results in an apparent hardening, which is beneficial for the stability of deformation. The stability of RA at a given temperature strongly depends on its carbon content, which, in AHSS, is not uniform but distributed. The aim of this study is to build a model that predicts the transformation as well as TRIP in a DP steel grade with RA. A physics-based kinetic model is presented that captures the transformation of retained austenite based on the thermodynamic driving force of the applied stress. A direct analytical estimate of transformation plasticity is provided, which is consistent with the kinetic model. Transformation kinetics is incorporated in a self-consistent, mean-field homogenization-based constitutive model. Finally, an indication of the effect of transformation of retained austenite on formability is given.

Effects of different laser welding parameters on the microstructural and mechanical properties of an advanced dual phase steel grade

Lazer kaynak prosesinde uygulanan parametrelerin kaynaklanmış yassı çelik ürün performansı üzerindeki etkilerinin belirlenmesi önemlidir. Bu çalışmada, ağırlıklı olarak otomotiv sektörü tarafından kullanılan ve ülkemizde talebi ithalat ile karşılanmakta olan ileri nesil çift fazlı bir çelik kalitesine farklı lazer kaynak parametreleri uygulanmıştır. 3 kW’lık bir fiber lazer sistemi kullanılarak yassı çelik levhalar alın kaynak yöntemi ile birleştirilmiştir. Bu çelik kalitesi için uygun olabilecek kaynak parametre aralıklarının gözlemlenmesi amacıyla, farklı lazer kaynak parametrelerinin mikroyapı ve mekanik özellikler üzerindeki etkileri araştırılmıştır. Çekme testleri, mikrosertlik ölçümleri ve metalografik çalışmalar gerçekleştirilerek kaynak bölgeleri, ısı tesiri altındaki bölgeler ve esas metal incelenmiştir. Lazer kaynak denemelerinin ikisinde tam kaynak nüfuziyetinin ve esas metalden yüksek akma ve çekme mukavemetinin sağlandığı levha çiftleri üretilebilmiştir. Diğer deneylerde, mikroyapısal inceleme bulguları mekanik performans sonuçlarını güçlü destekler biçimde, sert faz miktarı ve iç yapıda tane inceliğinin gevrek kırılma olgusunu tetiklediği görülmüştür. Sonuçlar lazer kaynak proses parametreleri ile ilişkilendirilmiş, optimum prosese yönelik parametre aralığı izlenmiştir. Bununla birlikte, lazer gücü ve lazer ilerleme hızının ve böylece toplam ısı girdisinin iyi kalite kaynak eldesi için optimize edilmesi gereken kritik parametreler olduğu doğrulanmıştır.

Numerical analysis of flow behaviour, grain size prediction and experimental verification of hot rolled ultralow carbon niobium microalloyed steel

AbstractNiobium bearing ultralow carbon micro alloyed dual phase steel grade steel with chemical composition of 0.045 wt.% carbon, 0.04 wt.% niobium, 0.59 wt.% chromium, 0.9 wt.% manganese, 0.28 wt.% silicon is investigated. The design of thermomechanical treatment needs knowledge about the flow behaviour at high deformation temperatures, to be used for this steel in industrial plants. Therefore, this work aims to predict, for the investigated steel, the flow behaviour and mean flow stress. A phenomenological constitutive model is established to derive the flow stress using the hyperbolic sinusoidal Arrhenius mathematical model. The relation between stress and strain with Zener‐Hollomon parameter is studied. Mean flow stress has been figured out by measurements taken from compact slab production plant log data using a constitutive model. The constitutive model is further verified by multiple hits of flat compression setup mode at BährTTS820 physical simulator. The tests were performed under specific thermochemical schedule simulating compact slab production plant. The results show good agreement between the calculated flow stress, Non‐recrystallization temperature and experimentally measured values obtained from the physical simulation. Ferrite grain size is modelled and predicted then validated by experimental rolled specimens in a practical hot strip mill.

Microstructural Evaluation and Influence of Welding Parameters on Electrode Plunge Depth in Resistance Spot Welded Dissimilar DP800HF/1200M Steel Joints

Advanced high strength steels (AHSS) are newly developed steels that has versatile mechanical properties. These steels enables to design low weight cars with high safety standards. Also, weight reduction in vehicles plays a significant role for saving fossil fuels which is limited and causes carbon emissions. Dual phase (DP) and Martensitic steels are prominent in AHSS family because they are inexpensive and has vast application areas. DP steels are used for general purpose applications and Martensitic steels are used for reinforcement parts in vehicles. In this study, high formable grade Dual phase steel with 800 MPa tensile strength and Martensitic steel with 1200 MPa tensile strength were welded with resistance spot welding technique which is the most widely practiced joining method in the industry. Electrode indentation depths, its effect on tensile-shear loads and microstructural characterizations were investigated. According to the results, the lowest tensile shear loads were acquired between both between 0-0.2 mm and between 0.85-1mm electrode plunge depths. Medial electrode plunge depths showed high tensile shear loads. Some welding defects were encountered including secondary phase formations, shrinkage voids, intergranular shrinkage gaps and vertical cracks in the weld nugget. It is find out that the weld defects were formed due to cooling gradient while solidifying, electrode force, and improper weld parameters.

Rapid alloy prototyping for strip steel development: DP800 steel case study

ABSTRACT Alloy development on a commercial scale in recent years has become much more costly and as such has adopted a more iterative approach, making small refinements while keeping within known limits. Laboratory-based rapid alloy development is therefore a key in pushing new grade boundaries. This work shows the rapid alloy development (RAP) facilities at WMG, including capability, sizes and time required for each stage of the process, which have been mapped against the relevant industrial process. A case study of a dual-phase steel grade DP800 has been used for this work. It has been shown that through knowledge of the strengthening mechanism of the alloy (2nd phase distribution for DP800) is critical with respect to mechanical properties and that specific changes and optimisation to the laboratory production casting and subsequent reduction allows for the final product microstructure and mechanical properties to match the production product. This gives confidence in upscalability.

Effect of Tempering on Stretch-Flangeability of 980 MPa Grade Dual-Phase Steel

In this study, the effect of tempering on the stretch-flangeability is investigated in 980 MPa grade dual-phase steel consisting of ferrite and martensite phases. During tempering at 300 oC, the strength of ferrite increases due to the pinning of dislocations by carbon atoms released from martensite, while martensite is softened as a consequence of a reduction in its carbon super-saturation. This strength variation results in a considerable increase in yield strength of the steel, without loss of tensile strength. The hole expansion test shows that steel tempered for 20 min (T20 steel) exhibits a higher hole expansion ratio than that of steel without tempering (T0 steel). In T0 steel, severe plastic localization in ferrite causes easy pore formation at the ferrite-martensite interface and subsequent brittle crack propagation through the highly deformed ferrite area during hole expansion testing; this propagation is mainly attributed to the large difference in hardness between ferrite and martensite. When the difference in hardness is not so large (T20 steel), on the other hand, tempered martensite can be considerably deformed together with ferrite, thereby delaying pore formation and hindering crack propagation by crack blunting. Eventually, these different deformation and fracture behaviors contribute to the superior stretch-flangeability of T20 steel.

Effect of Composition System on Microstructure and Properties of 1000-MPa Grade Cold Rolled DPSteel

1000 MPa grade cold-rolled duplex steel with 5 component systems was trial-produced in the laboratory. After hot rolling, pickling and cold rolling, the test steel was simulated continuous annealing on Gleeble-3800 thermal simulator. The microstructure and properties of 5 kinds of experimental steels after simulated annealing were analyzed. The results show that C and Mn elements are the basic alloy systems of all duplex steels. In the case of low Si element, the C element content of the 1000 MPa grade dual phase steel should be 0.15 % or higher, and the Mn element content should be higher than 1.8 %. Si element increased to the A3 point, while the amount of austenite was less in the two-phase region during annealing, and the volume fractionof martensiteafter the rapid cooling is insufficient; Nb element is a high-efficiency fine-grain strengthening element, which greatly improvedthe uniformity of the steel species. The fine-grained martensite islands have enhance the tempering stability of martensite and thereby to reduce the adverse strength effect from over-aging temperature.

Accelerating the Discovery of New DP Steel Using Machine Learning-Based Multiscale Materials Simulations

In recent years, the use of dual-phase (DP) steels by the automotive industry has been growing rapidly, motivated by government policies prompting the production of fuel-efficient vehicles. While it is of high interest for the transportation industry to design and discover different grades of DP steels exhibiting desirable mechanical properties, this requires exploring a large number of DP steel microstructure combinations. Expensive trial-and-error-based experimentations and multiscale materials simulations are two conventional approaches that have been widely adopted in the field of materials design and discovery. Yet, it is challenging to use such approaches for fast materials design and discovery when considering the computational and cost limitations, as it is computationally infeasible and intractable to use multiscale materials models to characterize the mechanical properties of millions of different microstructures. To address this major limitation in material design, a Gaussian process is developed to accelerate the discovery of the mechanical properties of different DP steels by evolving the microstructure parameters using a limited number of numerical simulations (using a multiscale materials model). A Gaussian process is a machine learning technique that is trained to find nontrivial correlations between a set of inputs (microstructure properties) to predict a desired output (mechanical property). The proposed Gaussian process not only accelerates the prediction of the desired mechanical properties of millions of multiscale materials simulations but also offers uncertainty quantification around its predictions. These merits make the Gaussian process a very reliable, robust, and practical solution for material design exploration. The proposed framework combining multiscale simulations and the Gaussian process is used to discover the microstructural design of DP steel with maximum tensile toughness. The results showed the effectiveness and robustness of the proposed method in comparison to benchmark methods.

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.

Shrinkage Void Formation in Resistance Spot Welds: Its Effect on Advanced High-Strength-Steel Weld Strength and Failure Modes

Among various defects formed in resistance spot welds of advanced high-strength steels (AHSSs), shrinkage void (cavity) is the least addressed topic. Although cavities are formed in numerous sizes and shapes, their influence on the strength and failure modes is yet to be investigated. The present study examines the effect of the cavity size and position on the peak tensile shear load and failure modes in 780 grade dual-phase steel used in automobile body structures. The size and position of the cavity in the nugget were varied carefully by altering the weld parameters and the sheet thickness. The cavity characteristics and its effect on the weld strength were evaluated from shear tensile tests and fracture surfaces. Results show that the size of the cavity increases with sheet thickness and nugget diameter. Further analyses revealed that the shear strength of the weld is influenced by the size and location of the cavity from the interface. A transition from plug-type failure to interfacial failure was observed with an increase in the cavity size. Consequently, the cavities reduce the load-carrying area of the nugget affecting the weld strength and failure mode. The effective nugget area, which is the actual load-bearing area of the nugget without any cavity inferred from the interfacial fracture surface, would be beneficial in determining the interfacial failures in resistance spot welds. Based on the results, a minimum nugget diameter of 5√t was proposed to obtain plug-type failures in AHSS spots with the cavity.

On the mechanical heterogeneity in dual phase steel grades: Activation of slip systems and deformation of martensite in DP800

We used micropillar compression experiments to study the plasticity of ferrite and martensite of two commercial dual phase steel grades (DP800). The activation of all three slip plane families, namely {110}, {112}, {123}, was observed in single crystalline ferrite pillars. They exhibit a comparable mean critical resolved shear stress (CRSS) of 147 ± 6, 143 ± 9, 146 ± 4 MPa for 3 µm pillars and are predominantly following Schmid´s law. A distinct size effect occurs when comparing the CRSS of 2 µm and 3 µm pillars. Martensite islands show uniform deformation and exhibit high compressive yield strength up to nearly 3 GPa. In most cases martensite pillars deform in an isotropic fashion without distinct slip traces. Despite the identical ultimate tensile stress of two steel grades their ferrite CRSS and martensite strength are largely different. It is found that the softer ferrite results in a lower macroscopic yield strength and a higher elongation to failure during macroscopic tensile testing. The results suggest that an increased local strain hardening capability suppresses global damage. The data provided here can serve as input parameter for crystal plasticity modeling.

In-situ EBSD study of deformation behaviour of 600 MPa grade dual phase steel during uniaxial tensile tests

This paper studied the plastic deformation behaviour of DP600 steel subjected to uniaxial tension, by means of in-situ EBSD technique. It provides experimental evidences and detailed insight into the microstructural aspects of plastic deformation. A phase identification method based on the band slope map of EBSD was adopted to differentiate martensite from ferrite. The results show that the plastic strain localization lies mainly in the ferrite grains, fracture could usually start in ferrite grains close to hard martensite grains. With the increase of strain, average misorientation angle decreased while the fraction of LAGBs increased. Average Taylor factor for the whole microstructure became higher at high strains due to work hardening process, and plastic deformation results in soft regions with zonal distribution parallel to the loading direction. In the undeformed state, the texture orientation (111)[01¯1] and (111)[11¯0] are the major components of the γ-fibre while (223)[11¯0] and (221)[11¯0] are the main components of α-fibre. The intensity of the α-fibre slightly decreased, while the intensity of the γ-fibre increased with increasing strain. Plastic deformation occurred in some grains which were subdivided into different regions due to the activation of different slip systems. The tensile axis orientation of the grain rotated gradually to the line link <100>−<101>, and lattice rotation within one single grain differs from regions to regions.

Effects of pre-annealing conditions on the microstructure and properties of vanadium-bearing dual-phase steels produced using continuous galvanizing line simulations

It is well known that the fracture behavior and weldability of steel often vary inversely with the carbon content. In the case of thin sheet intended for automotive applications, this means spot weldability and tensile ductility. However, the question arises, what is the reasonable maximum strength attainable in a low-carbon dual-phase (DP) steel under the constraints of: (1) with carbon levels at 0.1 wt%, (2) with the product (UTS × % TE) > 18,000 MPa, and (3) with processing on a simulated continuous hot-dipped galvanizing line. This study has focused on experimental compositions intended to meet DP steel grade properties in excess of 780 MPa UTS. This kind of advanced high-strength steel (AHSS) is an integral part of mass reduction programs for the body-in-white for the automotive industry. The family of steels investigated here is based on a low-carbon, aluminum-killed base steel containing Cr and Mo. In this study, the effects of the presence of V were studied. In addition, there were three processing variables investigated in terms of their respective effects on microstructure and mechanical properties: first, hot band coiling temperature; second, % cold reduction; and third, the thermal path through the CGL process. The thermal paths explored included both a standard galvanizing process and a new supercooling process; these were studied using a CGL simulation. This research revealed that all three variables can have a significant effect on the final microstructure and properties.