Papers by Keyword: Retained Austenite

Abstract: The authors have developed the texture measurement system at iMATERIA, which is the neutron diffractometers built in Materials and Life Science Experimental Facility (MLF) at J-PARC, Japan. The high flux of the incident beam and Time-Of-Flight method enabled the complete texture measurement within several minutes in case of steels. Since the neutron beam can transmit most of the materials, the measured texture represents the state of whole exposed volume. The multi-histogram analysis also enables to determine phase fractions in a multiphase material as well as the texture of each phase.

Abstract: Q&P annealing cycles with different partitioning conditions were performed on cold rolled 0.2C-2.22Mn-1.44Si-0.21Cr steel. An important influence of partitioning temperature and time on the evolution of retained austenite fraction was shown through the saturation magnetization measurements. Such effect of partitioning conditions was also observed on the evolution of mechanical behavior. The evolution of microstructure and mechanical properties with the partitioning conditions was analyzed. Mechanical stability of retained austenite as a function of partitioning time was also assessed. Finally, modeling of the obtained stress-strain curves was performed and some explanations of the observed tendencies between partitioning conditions and tensile properties were proposed.

Abstract: The effects of warm working on microstructural, retained austenite characteristics and shear deformation properties of 0.2C–1.5Si–1.5Mn–1.0Cr–0.2Mo TRIP-aided martensitic (TM) steel for applications to automotive frame and forging parts were investigated. When warm working at 550 °C and post cooling at 1 °C/s was conducted to the TM steel, volume fractions of retained austenite and martensite-austenite constituent phase increased and mixture matrix of ultra fine granular bainitic ferrite and fine bainitic ferrite lath was obtained, whereas microstructure of TM steel warm worked at 750 °C exhibited granular bainitic ferrite matrix. These were caused by the dynamic recrystallization and the promotion of bainitic transformation of austenite due to the worm forging at 550 °C with the post cooling rate of 1 °C/s. Maximum shear stress decreased and total shear displacement increased with decreasing working temperature in TM steel. These were caused by the effective strain induced transformation of a large amount of retained austenite and the refined matrix structure.

Abstract: The study addresses relationships between the microstructure and mechanical properties of thermomechanically processed carbide-free bainitic steels containing 3% and 5% Mn. A simulated thermomechanical processing using Gleeble equipment and thermomechanical hot strip rolling were applied to produce fine-grained mixtures of blocky-type and interlath metastable retained austenite embeded between bainitic ferrite laths. To monitor the transformation behaviour of retained austenite into strain-induced martensite interrupted tensile tests were applied. The identification of morphological features of retained austenite and strain-induced martensite was carried out using scanning electron microscopy (SEM) equipped with EBSD (Electron Backscatter Diffraction). The amount of retained austenite was determined by the EBSD technique. It was found that manganese content strongly affects mechanical stability of retained austenite resulting in a different degree of TRIP effect in the investigated alloys and subsequent mechanical properties of produced sheets.

Abstract: Casting of steel with different aluminum content was carried out. Thermo-mechanical process was carried out at 1200°C. A cross-sectional area reduction of 95% was done using hot forging hammer. Two regimes of thermomechanical processes were adopted in this study. First regime was intercritical annealing (between AC₁ and AC₃ to obtain 50% austenite and 50% ferrite) followed by rapid quenching in salt bath to the martensite zone for 15minutes to produce ferrite, martensite and carbon saturated austenite and to prevent the whole transformation of austenite to martensite. The second regime was reheating 50oC above AC₃, followed by rapid quenching in salt bath to the martensite zone (for 15 minutes) to get partially partitioned martensite and super saturated retained austenite. The initial tensile strength after hot forging is 1067MPa and 942MPa for alloys 1 and 2 respectively. The tensile strength after intercritical annealing is 1621MPa while after quenching from austenite to martensite zone with austenite partitioning is 2113MPa. Elongation after intercritical annealing and austenite partitioning is null due to bad shapes and distribution of ferrite with sharp corners.

Abstract: Martensitic high-carbon, high-strength bearing steel is used for rolling contact applications when high wear and fatigue resistance are required. Due to its high fatigue strength, SUJ2 is not used for only bearings but for shafts. The objective of this work is a clarification of the relationship between quenching times and retained austenite amount of SUJ2 steel. It was found that repeatedly induction heating increased the retained austenite amount, but did not change the Vickers hardness.

Abstract: Due to the exploration of gas and oil sources in remote regions with harsh environment, the pipe line designers are faced with the challenging conditions. Especially in the cases where displacement-controlled loads are the predominant design condition, such as ground movement, strain-based design is applied instead of stress-based design to build safer pipelines and to assure the integrity of the lines along their lifetimes. It requires steels to have a large strain hardening capacity, long uniform elongation, and good toughness to achieve a well-defined and sufficient plastic deformation. In order to fulfil the requirements, all the processing steps are supposed to be well designed, for example, the chemical composition, microstructural design, thermo-mechanical controlled process (TMCP) and heat treatment, etc. In general pipe line steel has a low carbon, high manganese content combined with a certain amount of micro-alloying elements as Nb, Ti and sometimes B. Using Thermo-mechanical rolling and cooling schedules excellent combinations of strength and toughness can be established. To successfully produce steel plates for strain based design pipe lines the actual heating rolling and cooling technologies must be developed further as well as the equipment to produce such steel. The paper gives an overview about modern plate mill equipment and new setup strategies for the production of heavy plates X70 to X100 as well as thethermo-dynamic simulations used during the development.

Abstract: The microstructure and mechanical properties of a medium manganese quenching and partitioning (Q&P) steel for automobile were investigated by optical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD) and mechanical property test. The grain size and recovery degree were greatly affected by annealing temperature normally. The result shows that the medium manganese steel after quenching and partitioning (Q&P) heat treatment exhibited good mechanical properties. The maximum tensile strength and yield strength was 1280MPa and 1421MPa at 600°C, respectively. Additionally, the product of strength and plasticity could reached to 40472MPa×% at 640°C. Annealing temperature also had a great influence on the volume of retained austenite which increases linearly with the rise of annealing temperature as well.

Abstract: Abstract. Excellent combination of mechanical properties makes bainitic steels very attractive for commercial application. The most potential benefit of bainitic steels is found in the lightweight design of car bodies. The chemical composition, productions process and the austenite state control the properties of those steels. To reach the target of the correct component design it is important to focus on transformation kinetics. High carbon steels have been investigated, which contain approximately 0.5% C, 1.5% Si, 1.5% Mn, 0.9% Cr and 1.5% Cu. It’s expected to form a carbide free bainitic microstructure due to the Si addition. The residual austenite in the microstructure of high carbon bainitic steels ensures the forming ability besides the high strength. Maximum tensile strength is 1650 MPa and elongation is around 30%. Those steels can be used to produce large scale components.

Abstract: The substitution of Si with Al in 0.2%C-1.5%Si-1.25%Mn-0.2%Cr ultrahigh strength transformation-induced plasticity (TRIP)-aided martensitic (TM) sheet steel improves galvanization. The effect of Al content on the microstructure and formabilities of the TM steel was therefore investigated. Replacement of Si with Al maintained the high volume fraction of the retained austenite and the high stretch-formability and stretch-flangeability, whereas it decreased the tensile strength. Complex addition of Si and Al yielded the best formabilities with 1.5 GPa tensile strength grade. The superior formabilities of Si-Al bearing TM steel were attributed to the strain-induced transformation of the metastable retained austenite and the relatively soft lath-martensite structure matrix. The former leads to plastic relaxation of the localized stress concentrations, thus suppressing void formation.