Papers by Keyword: Retained Austenite Stability

Abstract: In this paper, we report a lamellar-structured low-alloy transformation-induced plasticity (TRIP) steel; the microstructure of the steel consists of alternate lamellae of intercritical ferrite and reverted austenite on microscale, with the latter consisting of bainitic ferrite laths and retained austenite films on nanoscale. Such a microstructure was produced by a heat treatment process similar to that for producing conventional TRIP-assisted steels, i.e. intercritical annealing followed by austempering. Nevertheless, quenched martensite rather than a mixture of ferrite and pearlite was used as the starting structure for intercritical annealing to form austenite, and the resulting austenite was then transformed to bainite by austempering treatment. This steel exhibits much enhanced strength-ductility combinations as compared with those conventional polygonal-structured low-alloy TRIP steels.

Abstract: In the present study, the effects of ausforming on the bainitic transformation, microstructure and mechanical properties of a low-carbon rich-silicon carbide-free bainitic steel have been investigated. Results show that prior ausforming shortens both the incubation period and finishing time of bainitic transformation during isothermal treatment at a temperature slightly above the M_s point. The thicknesses of bainitic ferrite laths are reduced appreciably by ausforming; however, ausforming increases the amount of large blocks of retained austenite/martenisite and decreases the volume fraction of retained austenite. And accordingly, ausforming gives rise to significant increases in both yield and tensile strengths, but causes noticeable decreases in ductility and impact toughness.

Abstract: The effects of the intercritical annealing temperature and initial microstructure on the stability of retained austenite were investigated for a 0.1C-6Mn (wt-%) steel. Medium-Mn transformation-induced plasticity (TRIP) steels exhibit a strong dependence of their mechanical properties on the variation of intercritical annealing temperature. This behavior is strongly linked to the amount and stability of the retained austenite. Thus, interrupted tensile tests were used to examine the effect of annealing temperature on the stabilization of the retained austenite. Detailed microstructural investigations were employed to elaborate the effects of its chemical and mechanical stabilization. Furthermore, the final microstructure was varied by applying the batch annealing step to an initial non-deformed and deformed microstructure respectively. Retained austenite stability along with resulting mechanical properties of the investigated medium-Mn TRIP steel was significantly influenced as the amount and morphology of the respective phases altered as a consequence of both initial microstructure and applied intercritical annealing temperature.

Abstract: The forming at elevated temperatures for Advanced High Strength Steels (AHSS) opens up a new technology. The phase changes during warm deformation are the key to understanding the warm forming process. The desired microstructure and mechanical properties before and after warm forming have to be known in order to find optimal conditions for achieving good sheet formability and preferred material properties in service. In this work, the TRIP690 and DP780 steels are investigated under punch stretch test conditions in order to evaluate the temperature influence on neck formation and fracture occurrence at ambient and elevated temperatures 200oC, 400oC. Contact heat treatment was used for heating up the circular specimens. It was found that formability of the investigated steels was drastically reduced at a temperature of 400oC and brittle fracture occurred because of temper embrittlement. It is recommended to avoid steel tempering at this critical temperature.

Abstract: With the increasing demand for high performance engine or suspension components, bainitic steels are receiving interest as potential replacement of their quench and tempered counterparts. Indeed, for a number of mechanical components, ferrite pearlite microstructures are no longer sufficient in terms of mechanical properties. Bainitic steel grades allow production of hot-rolled bars or forged components exhibiting a homogeneous bainitic microstructure and achieving UTS up to 1200 MPa without the need for additional heat-treatments [1]. During tempering, these V-microalloyed bainitic steels exhibit unusual yield strength variations, with a very pronounced increase around 250-300 °C followed by the better known secondary hardening peak for temperatures around 600-650 °C. Indeed, after tempering at 250-300 °C, some of these steels exhibit an increase in yield strength of up to 200 MPa, concurrent with an increase in impact toughness of up to 25%. This, however, goes unnoticed if hardness measurements are used to characterize tempering. In the following, results are presented for three different bainitic steel grades, and the origins of the changes in mechanical properties are discussed.

Abstract: In this work we compare and contrast the stability of retained austenite during tensile testing of Nb-Mo-Al transformation-induced plasticity steel subjected to different thermomechanical processing schedules. The obtained microstructures were characterised using optical metallography, transmission electron microscopy and X-ray diffraction. The transformation of retained austenite to martensite under tensile loading was observed by in-situ high energy X-ray diffraction at 1ID / APS. It has been shown that the variations in the microstructure of the steel, such as volume fractions of present phases, their morphology and dimensions, play a critical role in the strain-induced transition of retained austenite to martensite.

Abstract: Transformation induced plasticity (TRIP) steels have complex multiphase microstructure composed of ferrite, bainite and retained austenite [1]. These metastable retained austenite can transforms into martensite during plastic deformation, which generates a TRIP effect resulting in excellent combination of high strength and ductility even at high strength level [2-5]. For this reason, the TRIP-aided steel sheets are suitable to fabricate automobile parts, as they can offer excellent formability without sacrifice the strength and safety requirement of the steel sheets. As a result, the development of TRIP-aided steels has been a very important issue in the automobile field.

Abstract: Two-stage heat treatment process which has guiding significance for continuous annealing TRIP steel producing was applied in this research. Different matrixes such as polygonal ferrite matrix, bainite ferrite matrix and annealed martenite matrix were obtained through different heat treatment processes. Compared the transformation-induced plasticity (TRIP) behaviors of three different tested samples，and the corresponding process for required product properties can be chosen. It was found that the needle-like retained austenite obtained in AMT steel was isolated from other microstructures while the retained austenite in the other two samples appeared to be equiaxed or network structure. The elongation, yield ratio and stability of retained austenite in AMT steel were all higher than that in PFT or BFT steels. BFT steel possesses highest tensile strength and lowest elongation while the yield ratio, RA content and carbon concentration in RA were all lowest for PFT steel.

Abstract: The micro-alloying concepts have been widely used as a way to optimize the microstructure evolution and improve mechanical properties of conventional constructional steels. In the current study, the effect of micro-alloying on the properties of a multi-phase TRIP steel is analyzed. The micro-alloying of TRIP steel was found to lead to an increase of the yield stress rather than the tensile strength. A physical metallurgical explanation of the effect is proposed.

Abstract: The precise characterization of the multiphase microstructure of low alloyed TRIP steels is of great importance for the interpretation and optimisation of their mechanical properties. In-situ neutron diffraction experiment was employed for monitoring of conditioned austenite transformation to ferrite, and also for retained austenite stability evaluation during subsequent mechanical loading. The progress in austenite decomposition to ferrite is monitored at different transformation temperatures. The relevant information on the course of transformation is extracted from neutron diffraction spectra. The integrated intensities of austenite and ferrite neutron diffraction profiles over the time of transformation are then assumed as a measure of the volume fractions of both phases in dependence on transformation temperature. Useful information was also obtained on retained austenite stability in TRIP steel during mechanical testing. The in-situ neutron diffraction experiments were conducted at two different diffractometers to assess the reliability of neutron diffraction technique in monitoring the transformation of retained austenite during room temperature tensile test. In both experiments the neutron investigation was focused on the volume fraction quantification of retained austenite as well as on internal stresses rising in structure phases due to retained austenite transformation.