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SCOPUS: ch.b, info:eu-repo/semantics/published

Dual phase (DP) steels are widely used for their excellent combination of strength-ductility-cost perfor- mance. However, several forming and/or structural applications are affected by cracking resistance issues. We show here that DP steels with a ‘Thomas-fibers’ type platelet martensite morphology exhibit sig- nificantly larger cracking resistance compared to corresponding equiaxed microstructures. The superior cracking resistance is demonstrated both for thin and thick specimens processed with different marten- site volume fractions and thicknesses. The cracking of thin sheets involves complex intermingling of work spent for crack tip necking and for material fracture as quantified using the essential work of fracture method, complemented with fractography and microstructure characterization. The better cracking resis- tance is shown to originate from a remarkable resistance to damage nucleation related to the alignment of the platelets upon deformation and from their small size. This finding offers a new path to optimize the fracture toughness of DP steels without compromising the strength and/or changing the chemical composition.

Martensite volume fraction, composition and grain size are the known primary factors controlling the mechanical behavior of ferrite-martensite dual-phase steels. Recently, excellent performances of dual-phase steels with a fibrous microstructure have been reported. However, the precise role of martensite morphology and orientation has not been thoroughly elucidated yet. This study develops a two-scale micromechanical modeling strategy in order to investigate the effect of particle morphology and orientation on the elastoplastic behavior of dual-phase steels. Finite element simulations are carried out on 3D periodic unit cells, each having a given orientation and volume fraction of spheroidal particles. The overall response is obtained by averaging the response of grains with different orientations, thus bypassing the need for costly full-field simulations on representative volume elements of realistic microstructures. A detailed parameter study systematically investigates the effect of particle morphology and orientation at grain level and at grain assembly level. While particle morphology and orientation effects lead to significant differences at grain level in terms of strain hardening behavior and back-stress development, the impact of the phase morphology at the homogenized multigrain level is almost negligible up to the onset of necking. However, the mechanical fields at the micro-scale are considerably influenced by both particle morphology and orientation, and are expected to largely impact the damage behavior through, among others, generating large grain-to-grain heterogeneities.

The enhancement of the fracture toughness is essential for opening the possible range of applications of advanced high-strength steels, while the focus in the literature is primarily on the strength–ductility compromise. A high fracture toughness is indeed needed for energy absorbing components as well as to limit edge cracking sensitivity during part forming. This study investigates the tensile properties and the fracture toughness of various quenched and partitioned microstructures. The fracture resistance is evaluated using double-edge notched tension tests. While the uniform elongation continuously increases with the retained austenite (RA) fraction, the fracture toughness shows a maximum at intermediate RA content. For the highest amount of RA, the relatively low fracture toughness is mainly attributed to the formation of brittle necklace of fresh blocky martensite in the fracture process zone due to a high stress triaxiality, inducing an intergranular fracture mode. For intermediate RA fraction, the RA morphology evolves from blocky to film type, leading to a transition from intergranular to ductile fracture mode, and the RA-to-martensite transformation contributes to a higher total work of fracture compared to tempered martensitic steel. A proper control of both the amount and morphology of RA during microstructure design is thus essential to generate the best compromise between tensile properties and fracture toughness.

A two-step quenching and partitioning steel exhibits ductile fracture under uniaxial tension and unexpectedly intergranular brittle fracture in the case of pre-cracked configuration. A gradual retained austenite-to-martensite transformation because of low stress triaxiality under uniaxial tension, leads to ductile fracture with voids formation between fresh and tempered martensite resulting from the large strain partitioning and localization. When the retained austenite fast transforms to martensite in the near crack tip region because of high stress triaxiality, intergranular fracture is promoted by the high maximum principal stress level and by the formation of brittle martensite necklace along tempered martensitic packet boundaries.

Dual-phase steels have long been used in the automotive industry for their excellent mechanical properties in terms of strength and ductility balance combined to a low processing cost. The good compromise between strength and ductility results from the very different properties of the constituent phases, namely ductile ferrite and hard martensite. In contrast with the plastic flow properties, the fracture toughness of dual-phase steels (quantified by K Ic or J Ic ) has been far less investigated. Common values of the fracture toughness are -2 around 100 kJ.m or lower. However, a minimum level of fracture toughness is required to prevent the propagation during forming operations of small edge damage or cracked zones induced by cutting. Therefore, unravelling the relationship between fracture toughness, microstructure and damage mechanisms is essential to develop advanced steels with superior forming ability. Furthermore, reaching superior fracture toughness could open to other potential applications. Dual-phase steels are usually processed following an intercritical annealing which generally leads to equiaxed martensite particles. An alternative heat treatment, consisting of a double annealing as first proposed N.J. Kim and G. Thomas [1], brings about martensite particles in the form of platelets. A recent study on bulk samples of such steels shows that this microstructure can potentially lead to a very high fracture toughness, while retaining good properties in terms of strength and ductility [2]. In this work, rather oriented towards thin sheets, the Essential Work of Fracture (EWF) method [3] is used to quantify the work per unit area spent in the fracture process zone by separating it from the total work expended for material failure. EWF -2 values in excess of 300 kJ.m have been found on platelet-like microstructure steels confirming their interesting resistance to the propagation of a crack. Equiaxed microstructures are investigated as well and the impact of ma

Dual-Phase steels have long been used in the automotive industry for their excellent mechanical properties in terms of strength and ductility balance combined to a low processing cost. The good compromise between strength and ductility results from the very different properties of the constituent phases, namely ductile ferrite and hard martensite. In contrast with the plastic flow properties, the fracture toughness of Dual-Phase steels (quantified by KIc or JIc) has been far less investigated. Common values of the fracture toughness are around 100 kJ.m-2 or lower. However, a minimum level of fracture toughness is required to prevent the propagation during forming operations of small edge damage or cracked zones induced by cutting. Therefore, unravelling the relationship between fracture toughness, microstructure and damage mechanisms is essential to develop advanced steels with superior forming ability. Furthermore, reaching superior fracture toughness could open to other potential applications. Dual-Phase steels are usually processed following an intercritical annealing which generally leads to equiaxed martensite inclusions. An alternative heat treatment, consisting of a double annealing as first proposed N.J. Kim and G. Thomas [1], brings about martensite inclusions in the form of platelets. A recent study on such steels shows that this microstructure can potentially lead to a very high fracture toughness, while retaining good properties in terms of strength and ductility [2]. The general objective of this research is to investigate the fundamental damage mechanisms governing the fracture toughness of Dual-Phase steels. Our approach is based on the processing of microstructures in which parameters are varied one by one. In particular, both equiaxed and platelet-like microstructures are investigated in the form of thin sheets. Experimentally, the Essential Work of Fracture (EWF) method [3] is used to quantify the work per unit area spent in the fracture process zo

Model dual-phase microstructures were developed to decouple the effect of martensite volume fraction and martensite hardness on the plastic behavior of dual-phase steels. The martensite volume fraction ranges from 11% to 37%, involving two levels of martensite hardness. The yield strength and tensile strength increase with increasing martensite volume fraction, while the uniform elongation decreases. The martensite hardness has a weak impact on the initial yield strength, but it significantly affects the flow behavior for sufficiently large martensite volume fraction. Increasing the hardness of the martensite leads to higher tensile strength combined with only a limited impact on uniform elongation, resulting in an improved strength/ductility balance. The experimental results are successfully captured using finite element based micromechanical analysis. Among others, periodic cell calculations show very good predictive capabilities of the overall plastic response when the stage-IV hardening of the ferrite is taken into account. Our numerical analysis reveals that an accurate description of the elasto-plastic behavior of the martensite is a key element to rationalize the mechanical behavior of DP steels. This modeling approach provides a framework for designing dual-phase steels with optimized plastic flow properties.

The austenitization from a spheroidized microstructure during intercritical annealing was studied in a Fe-0.1C-3.5Mn alloy. The austenite grains preferentially nucleate and grow from intergranular cementite. The nucleation at intragranular cementite is significantly retarded or even suppressed. The DICTRA software, assuming local equilibrium conditions, was used to simulate the austenite growth kinetics at various temperatures and for analyzing the austenite growth mechanism. The results indicate that both the mode and the kinetics of austenite growth strongly depend on cementite composition. With sufficiently high cementite Mn content, the austenite growth is essentially composed of two stages, involving the partitioning growth controlled by Mn diffusion inside ferrite, followed by a stage controlled by Mn diffusion within austenite for final equilibration. The partitioning growth results in a homogeneous distribution of carbon within austenite, which is supported by NanoSIMS carbon mapping.

Dual-Phase steels have long been used in the automotive industry for their excellent mechanical properties in terms of strength and ductility balance combined to a low processing cost. The good compromise between strength and ductility results from the very different properties of the constituent phases, namely ductile ferrite and hard martensite. In contrast with the plastic flow properties, the fracture toughness of Dual-Phase steels (quantified by KIc or JIc) has been far less investigated. Common values of the fracture toughness are around 100[kJ.m-2] or lower; but seldom exceed the 200[kJ.m-2]. However, a minimum level of fracture toughness is required to prevent the propagation during forming operations of small edge damage or cracked zones induced by cutting. Therefore, unravelling the relationship between fracture toughness, microstructure and damage mechanisms is essential to develop advanced steels with superior forming ability. Dual-Phase steels are usually processed following an intercritical annealing which generally leads to equiaxed martensite inclusions. An alternative heat treatment, consisting of a double annealing as first proposed N.J. Kim and G. Thomas [1], brings about fibrous martensite inclusions. A very recent study on such steels shows that this fibrous microstructure can potentially lead to a very high fracture toughness, while retaining good properties in terms of strength and ductility [2]. The general objective of this research is to investigate the fundamental damage mechanisms that govern the fracture toughness of Dual-Phase steels. Our approach is based on the processing of microstructures in which parameters are varied one by one. In particular, both equiaxed and fibrous microstructures are investigated in the form of thin sheets. Experimental works as well as numerical calculations are used to study the behaviour of such steels. Experimentally, the characterization using a serial sectioning reveals that what seemed to be fibrous ma

Dual-Phase steels have long been used in the automotive industry due to their excellent mechanical properties in terms of strength and ductility, as well as their low processing cost. The good compromise between strength and ductility results from the very different properties of the constituent phases comprising ductile ferrite and hard martensite. In contrast with their plastic flow properties, the fracture toughness of Dual-Phase steels (quantified by KIc or JIc) has been far less investigated. Common values of the fracture toughness are around 100[kJ.m-2] or even lower; but seldom exceed the 200[kJ.m-2]. However, a minimum level of fracture toughness is required to prevent the propagation of small edge damage or cracked zones induced by cutting. Therefore, unravelling the relationship between fracture toughness, microstructure and damage mechanisms is essential to develop advanced steels with superior forming ability. Dual-Phase steels are usually processed following an intercritical annealing which generally leads to equiaxed martensite inclusions. An alternative heat treatment, consisting of a double annealing first proposed N.J. Kim and G. Thomas [1] brings about fibrous martensite inclusions. A very recent study on such steels shows that this fibrous microstructure can potentially lead to a very high fracture toughness, while retaining good properties in terms of strength and ductility [2]. The general objective of this research is to investigate the fundamental damage mechanisms that govern the fracture toughness of Dual-Phase steels. Our approach is based on the processing of microstructures in which parameters are varied one by one. In particular, both equiaxed and fibrous microstructures were investigated in the form of thin sheets. The Essential Work of Fracture (EWF) method [3] was used to quantify the work per unit area needed at the crack tip for material failure separating it from the total work expended for material failure. An extension of the EWF

A model to predict the evolution of dislocation density following hot deformation is presented in this article. The model is validated by stress relaxation experiments on austenite at various temperatures. It is found that the activation energy for self-diffusion is the rate-controlling parameter determining the evolution of dislocation density, and hence the recovery rate. A methodology to control the softening experienced by high-temperature alloys is proposed.