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.

Mechanical Behavior Evaluation of Tempcore and Hybrid Reinforcing Steel Bars via a Proposed Fatigue Damage Index in Long Terms

As it is widely known, corrosion constitutes a major deterioration factor for reinforced concrete structures which are located in coastal areas. This phenomenon, combined with repeated loads and, especially, intense seismic events, negatively affect their useful service life. It is well known that the microstructure of steel reinforcing bars has a significant impact either on their corrosion resistance or on their fatigue life. In the present manuscript, an effort has been made to study the effect of corrosive factors on fatigue response for two types of steel reinforcement: Tempcore steel B reinforcing bars and a new-generation, dual-phase (DP) steel F reinforcement. The findings of this experimental study showed that DP steel reinforcement’s rate of degradation due to corrosion seemed apparently lighter than Tempcore B with respect to its capacity to bear repeated loads to a satisfactory degree after corrosion. For this purpose, based on a quality material index that characterizes the mechanical performance of materials, an extended damage material indicator for fatigue conditions is similarly proposed for evaluating and classifying these two types of rebars in terms of material quality and durability. The outcomes of this investigation demonstrated the feasibility of fatigue damage indicators in the production cycle as well as at different exposure times, once corrosion phenomena had left their mark in steel reinforcement.

The fracture toughness of martensite islands in dual-phase DP800 steel

In situ microcantilever bending tests were performed on martensite islands in a dual-phase (DP) steel to extract the fracture toughness of martensite at the microscale and to understand damage initiation during forming of DP steels. All microcantilevers were produced through FIB milling. The martensite islands do not exhibit linear elastic brittle fracture; instead, significant ductile tearing is observed. The conditional fracture initiation toughness extracted by definition and by Pippan’s transfer criterion is Ki = 6.5 ± 0.4 MPa m1/2 and Ki,2% = 10.1 ± 0.3 MPa m1/2, respectively. The obtained value is well-represented by the strength-toughness trend of other ferritic steel grades. Considering the yield stress of the same martensite island, we found that crack initiation can occur only in very large martensite islands or in a banded or agglomerated martensite structure. Graphic abstract

Consideration of Magnetic Measurements for Characterisation of Ferrite–Martensite Commercial Dual-Phase (DP) Steel and Basis for Optimisation of the Operating Magnetic Field for Open Loop Deployable Sensors

The magnetic properties of commercial dual-phase (DP) steels (DP600, DP800 and DP1000 grades) were evaluated using initial permeability, incremental permeability and coercivity and correlated with the key microstructural differences between the grades. The ferrite grain sizes and ferrite fractions have been compared with the magnetic parameters obtained from minor and major magnetisation loops within each DP grade. It has been revealed that the incremental permeability increases with the applied magnetic field amplitude to reach a peak and then drops at a higher magnetic field, with the values being different for the three DP grades at a lower field and converging to a similar permeability value at the high field. The effects of ferrite grain size and phase fraction on the incremental permeability are considered, and it has been shown that the influence of ferrite grain boundaries on magnetic permeability is more dominant than the effect of ferrite fraction in commercial DP steel samples. An analysis of the correlation between coercivity and initial permeability with tensile strength shows that the initial permeability provides a slightly better prediction of strength for the steels examined, which is believed to be due to the fact that a combination of reversible and irreversible domain components affect the coercivity value, while the initial permeability is predominantly affected by reversible domain movements. Based on the trend between incremental permeability and applied magnetic field and the commercial EM sensor (EMspec) operating parameters, the effect of lift-off and hence magnetic field strength on the sensitivity to DP steel properties can be assessed.

Effect of Martensite Morphologies on Corrosion in 5% H2SO4 Solution of Borided X70 Dual Phase Steel

In the present investigation, some electrochemical properties of dual phase X70 steels with different martensite morphologies which have undergone boriding were studied. To obtain a variety of martensite morphologies, Direct Quenching (DQ), Intermediate Quenching (IQ) and Step Quenching (SQ) heat treatments were applied at an intercritical annealing temperature (IAT) of 760℃. The treatment (DQ) allowed the formation of fine martensite evenly distributed in the ferrite matrix. (IQ) treatment showed the formation of martensite along the ferrite / ferrite grain boundaries. In contrast, treatment (SQ) induced the formation of a banded morphology of martensite and ferrite. The realization of borides on X70 (DP) steel was carried out in a powder mixture containing 5% of B4C as source of boron, 5% of NaBF4 as activator and 90% of SiC as diluent at 950℃ for a period of time from 4 h. The corrosion behavior of X70 (DP) steel has been explored by the Tafel extrapolation method in a 5 wt. % H2SO4 solution. The corrosion resistance of steel which has undergone boriding (BDP) is higher than that of steel which has not undergone it (DP).

Preventing Nugget Shifting in Joining of Dissimilar Steels via Resistance Element Welding: A Numerical Simulation.

Joining the advanced high strength steels and the conventional steels is a critical issue for the manufacturing of lightweight vehicles. Resistance element welding (REW) is an emerging joining method for dissimilar metals and alloys by applying an auxiliary rivet-like resistance element in resistance spot welding (RSW). In this study, an electrical-thermal-mechanical coupled REW model for high-strength dual-phase (DP) steel and Q235 steel was developed by considering contact resistances as functions of temperature and surface contacting area. The results show that the welding element in REW serves to concentrate the current flow and thus Joule heat generation at the faying interface between the element and workpiece. For welding DP600 and Q235 workpieces with a small thickness ratio (≤0.4) or a high electrical resistivity ratio (≥3), REW could effectively mitigate nugget shifting between workpieces and reducing the thermal excursion to electrode as compared to RSW. Adding well-designed insulation layers in REW could further concentrate the current within the welding element, and enables a large-sized nugget at a lower current. This study is significant because it provides a better understanding to the electrical-thermal-mechanical behaviors with interfacial contacts in REW and contributes to its further advance.