In the present study, the fracture toughness of hardened and tempered powder metallurgical (PM) high-speed steel ASP 2030 was investigated using notched and unnotched bending specimens and the finite element method. The normal flexural strength of notched and unnotched specimens marquenched by austenitizing at 1150, 1170, and 1185°C, followed by quenching to room temperature is measured after triple tempering at 560°C for 2 h. The finite element method (FEM) analysis is performed to observe the true stress distribution and calculate the critical fracture stress in the specimens under the experimental conditions of the bending test. The microstructural features of the specimens were investigated by X-ray diffraction (XRD) and a field emission scanning electron microscope (FESEM) with an electron backscatter detector (EBSD). No retained austenite was detected in the tempered specimens, and according to the results of the EBSD analysis and XRD tests, the microstructure of the matrix consists of martensitic ferrite laths. It can be observed that with the increase of austenitizing temperature from 1150 to 11850C, the normal flexural strength of the specimens decreases. The decrease in flexural strength of the specimens is due to the increase in the prior austenite grain size and consequently the martensitic ferrite laths after tempering. In addition, as the austenitizing temperature increases, the volume fraction of the undissolved carbides decreases, which causes the size of the undissolved carbides to increase and the flexural strength to decrease. According to FEM, the critical crack length calculated from the critical fracture stress is approximately equal to the average diameter of undissolved carbides.

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This study presents the fracture toughness improvement of the powder metallurgical high speed steel ASP2030 by replacing the martensitic matrix with a bainitic one. For this purpose, the fracture toughness of the steel is compared in both the austempered and marquenched states. The heat treatment of the samples was carried out by austenitizing in the range of 1150-1185C followed by either austempering at 235C or marquenching processes. Triple tempering at 560 C for 2h was also performed on all specimens. Fracture toughness measurements were performed on circumferentially notched tensile specimens. The microstructural features of the specimens were investigated using X-ray diffraction, optical and electron microscopy, and electron backscatter analysis. The results showed that the fracture toughness is sensitive to small changes in austenitizing temperature at both heat treatment conditions. By increasing the austenitizing temperature from 1150 to 1170C, the fracture toughness increases and then decreases with a further increase in temperature to 1185C. The results suggest that the changes in fracture toughness are due to the simultaneous effects of the volume fraction of undissolved carbides and the lattice microstrain values at different austenitizing temperatures. It can be concluded that the fracture toughness increases when the martensitic matrix is replaced by a bainitic structure. This improvement is related to finer bainitic laths width and also to the larger spacing between the particles of the undissolved carbides.

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This work presents an advanced crystal plasticity model for the simulation of the mechanical behavior of multiphase advanced high-strength steels. The model is based on the Visco-Plastic Self-Consistent (VPSC) model and uses information about the material’s crystallographic texture and grain morphology together with a grain constitutive law. The law used here, based on the work of Pantleon, considers how dislocations are created and annihilated, as well as how they interact with obstacles such as grain boundaries and inclusions (carbides). Additionally, strain rate sensitivity is implemented using a phenomenological expression derived from literature data that does not require any fitting parameter. The model is applied to the study of two bainitic steels obtained by applying different heat treatments. After fitting the required parameters using tensile experiments in different directions at quasi-static and high strain rates, formability properties are determined using the model for the performance of virtual experiments: uniaxial tests are used to determine r-values and stress levels and biaxial tests are used for the calculation of yield surfaces and forming limit curves.@en

The third generation of advanced high strength steels shows promising properties for automotive applications. The macroscopic mechanical response of this generation can be further improved by a better understanding of failure mechanisms on the microstructural level and micro-mechanical behavior under various loading conditions. In the current study, the microstructure of a multiphase low silicon bainitic steel is characterized with a scanning electron microscope (SEM) equipped with an electron backscatter diffraction detector. A uniaxial tensile test is carried out on the bainitic steel with martensite and carbides as second phase constituents. An extensive image processing on SEM micrographs is conducted in order to quantify the void evolution during plastic deformation. Later, a new post-mortem electron backscatter diffraction-based method is introduced to address the correlation between crystallographic orientation and damage initiation. In this multiphase steel, particular crystallographic orientation components were observed to be highly susceptible to micro-void formation. It is shown that stress concentration around voids is rather relaxed by void growth than local plasticity. Therefore, this post-mortem method can be used as a validation tool together with a crystal plasticity-based hardening model in order to predict the susceptible crystallographic orientations to damage nucleation.@en

Global warming, continuous demand on energy from fossil fuels as a limited natural resource, and costumers’ high expectations regarding product quality are three global challenges that the automotive industry is facing. Fuel consumption reduction has a significant impact on preserving fossil fuels, lowering fossil fuel dependency and the CO2 emissions that result in global warming. Weight reduction of car bodies, as so called Bodies-In-White (BIW), is one possible solution that the automotive industry can invest on. However, there are other conflicting parameters to weight reduction such as passenger safety and formability, which need to be considered simultaneously...@en

Bainitic steels, as a third generation of advanced high strength steels, are potential steel grades for automotive applications. Two grades of bainitic steels with low and high silicon content, with three different thermal treatments per grade and therefore different second phase constituents, are examined under quasi-static and high strain rate deformations. Microstructures are studied by advanced characterization techniques, including X-ray diffraction and scanning electron microscope equipped with an electron backscatter diffraction detector. Subsequently, the quasi-static and dynamic mechanical responses of the steels are correlated to the microstructures. A positive effect of the strain rate is observed for all the examined materials: when the strain rate is increased, both the tensile stress and deformation levels increase, thus also the energy absorption capacity. However, it is shown that the higher the fraction of second phase constituents, the lower the effect of strain rate becomes. In addition, the grain size directly correlates to the strain rate effect too. The phenomenological hardening model of Johnson-Cook is used to simulate the quasi-static and dynamic flow behaviors, allowing to quantify the strain rate sensitivity for each material. A comprehensive literature survey on the strain rate sensitivity of various steel grades reveals that steels with higher strength demonstrate a lower strain rate sensitivity factor. This trend can be approximated by a power law function which clearly is followed by the materials under consideration in this study.

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Multiphase bainitic steels, as a third generation of advanced high strength steels, show promising properties for automotive applications. Understanding the micro-mechanisms of damage initiation during plastic deformation is a key to further mechanical properties enhancement. The topological effect of martensite as a second phase constituent on local damage nucleation activity is significant. This effect has been studied in two different martensite banded microstructures produced by two various annealing cycles. The postmortem damage analysis by a scanning electron microscope on uniaxial loaded samples, revealed more damage nucleation along the dispersed and fragmented martensite phase within martensite banded regions. More pronounced strain partitioning was observed in coarse bainitic grains between adjacent martensite blocks. It is shown that the fracture strain is not controlled by local damage activities, implying that earlier damage initiation or an increased volume fraction of voids does not give rise to a reduced ductility.@en

Recently, the third generation of advanced high strength steels (AHSSs) show promising properties for automotive applications. The improvement of macroscopic mechanical performance is not feasible without a deep understanding of the micromechanical behavior and failure micro-mechanisms involved during its response under various loading conditions. In this study, a uniaxial tensile test is conducted on a low silicon bainitic steel with second phase constituents (martensite and carbides). A comprehensive image processing on SEM micrographs is performed in order to quantify the damage evolution as a function of plastic deformation. A new methodology is examined to address the correlation between crystallographic orientation and damage initiation. In this multiphase steel, it appears that orientation dependence of damage initiation is blurred by the presence of different phases and hence there is not an obvious preferential orientation from where damage has initiated.@en