No abstract available

[en] A single X-ray diffraction scan is effectively used for identifying and evaluating deformation-induced transformation in 304 austenitic stainless steel. Variations in grain size influence surface constraint and hence the through-thickness transformation response. The initial stage of transformation in this steel is most likely dominated by ε-martensite formation

[en] Highlights: • Smaller PAGS is generally beneficial for mechanical performance of Al-Si coated PHS. • Tensile fracture/ductility is independent of austenitizing conditions of these PHS. • Quasi-cleavage occurs under plane strain in PHS austenitized at high temperatures. • Bendability/toughness decreases for Al-Si coated PHS austenitized at high temperatures. • Both Al-Si coating and martensite microstructure affect fracture behavior. -- Abstract: The influence of the austenitizing parameters on the microstructure and mechanical behavior of Al-Si coated press hardened 22MnB5 steel was evaluated in this study. Increasing the austenitizing temperature and hold time resulted in microstructural homogenization, an increase in the prior austenite grain size (PAGS), and increased thickness of the substrate-coating interdiffusion layer. The mechanical properties were evaluated using smooth-sided tensile testing, double edge-notch tensile testing, and free bend testing. Refinement of the PAGS was, in general, beneficial for the mechanical performance of the press hardened steel (PHS); i.e. strength, notch displacement, maximum bending load, and bend angle at maximum load all generally increased with decreasing PAGS. The stress and strain state experienced during each type of test influenced the sensitivity of mechanical and fracture behavior of the PHS to austenitizing conditions. Furthermore, the thickening of the substrate-coating interdiffusion layer, composed of brittle intermetallic phases, at higher austenitizing temperature negatively influenced bendability of the PHS. Ductile fracture was commonly observed in the steel matrix composed of a lath martensitic structure. However, quasi-cleavage fracture was observed for the martensitic matrix with the largest PAGS, which implies a loss of toughness, possibly because of the increased substructure size.

[en] The effects of intercritical annealing, before and after cold rolling, on retained austenite volume fractions, austenite stability, and tensile properties of a 7.39Mn-0.14C-1.55Al-0.2Si (wt pct) medium Mn sheet steel were evaluated. The extent of hot band annealing at 650 °C prior to cold rolling is shown to be an important parameter to consider in the development of processing histories leading to high strength–ductility combinations required for third-generation advanced high-strength steels.

[en] Solubility products for molybdenum carbide and tungsten carbide are absent from the literature despite their importance in secondary hardening steels and in microalloyed steels. Equilibrium solubility products were calculated for MoC, Mo₂C, WC and W₂C in ferrite and austenite. Molybdenum and tungsten carbides exhibit greater solubility than traditional microalloy carbides in iron and their predicted solution behavior is similar to that of other Group VI transition metal carbides. The calculated solubility products are consistent with precipitation behavior during secondary hardening in steels.

[en] A model is proposed to predict austenite stabilization through manganese partitioning between ferrite and austenite. The model predicts retained austenite fractions as a function of intercritical annealing temperature based on equilibrium phase fractions and solute contents. Thermodynamic data are used to calculate the M_s temperature of the enriched austenite in order to predict fresh martensite formation upon cooling to room temperature. An intercritical annealing temperature resulting in the greatest retained austenite fraction is predicted which correlates to some experimental observations.

[en] A multi-modal characterization technique, which combines nanoscale secondary ion mass spectroscopy (Nano-SIMS) with a spatial resolution of ∼100 nm and electron back scatter diffraction (EBSD) to determine carbon distributions in austenite and martensite in a quenched and partitioned (Q&P) Fe–0.29C–2.95Mn–1.59Si steel is presented. Significant carbon enrichment of austenite was measured with decreased levels of carbon in martensite, supporting the carbon partitioning mechanism. Fresh untempered martensite could be identified, and different degrees of enrichment were observed for blocky and lath austenite

[en] Highlights: • A model was proposed to predict tensile properties of intercritically annealed medium Mn TRIP steels. • Phase fraction and martensite transformation kinetics were calculated and used as input parameters to the model. • Martensite transformation kinetics under tensile deformation were predicted from austenite chemical composition. • Predicted evolution of tensile properties with annealing temperature showed good correlation with experimental results. • Maximum UTS×UE values and greatest retained austenite fractions were obtained at the temperature of cementite precipitation. Tensile properties were predicted for intercritically annealed 4.5 wt% Mn steels with varying Al additions up to 1.3 wt%. Intercritically annealed phase fractions and retained austenite chemical compositions were obtained using a retained austenite prediction model assuming ortho-equilibrium alloy partitioning. The martensite transformation kinetics during tensile deformation were also estimated from the predicted retained austenite chemical compositions. Predicted phase fractions and martensite transformation kinetics were incorporated in a composite model for multi-component systems. Predicted tensile strength and uniform elongation values were compared to the experimental data obtained from the steels annealed at temperatures ranging from 550 to 725 °C for 1 h followed by water quenching.

[en] Quenching and partitioning (Q and P) has been developed as a novel steel heat treatment to produce advanced high-strength microstructures consisting of a martensitic matrix containing significant amounts of retained austenite. Austenite stabilization is hypothesized to result from decarburization of the martensite and transport into the austenite. Differential scanning calorimetry was employed to study Q and P microstructures. Two exothermic events were observed when heating a Q and P sample from room temperature to 600 deg. C. An activation energy suggesting a mechanism controlled by carbon diffusion in bcc iron is obtained for the first peak which is believed to be associated with carbon partitioning. The second peak is believed to be associated with austenite decomposition.

[en] Electron backscatter diffraction (EBSD) measurements were performed to investigate the bainitic ferrite microstructure in low-carbon, microalloyed steels with varying C and Mn contents. Fully austenitized samples were isothermally heat treated at temperatures ranging from 450 to 550 °C to form bainitic ferrite. The bainitic ferrite microstructures and boundary characteristics obtained from the EBSD measurements were analyzed based on an inferred Kurdjumov-Sachs (K-S) orientation relationship. The heat treated samples exhibit a microstructure composed of laths and the lath aspect ratio tends to increase at lower isothermal heat treatment temperatures. High fractions of boundary misorientation angles below 5° are observed, which are due to lath boundaries in the microstructure. Additionally, misorientations of approximately 7°, 53° and 60° are observed, which are related to the sub-block, packet, and block boundaries, respectively. With decreasing isothermal heat treatment temperature, there is an increase of block boundaries; these boundaries are intervariant boundaries between different blocks within a packet, most of which have the misorientation angle of 60°. The specimens with a higher carbon level contained increased length of block boundaries, whereas the addition of Mn moderated the dependence of block boundary length on the heat treatment temperature within the experimental temperature range. Meanwhile, the length of intervariant boundaries of both packet and sub-block character did not vary much with heat treatment temperature and alloy composition.