Papers by Keyword: Advanced High Strength Steel

Abstract: In-situ high-energy synchrotron X-ray diffraction was employed to track phase evolution in a medium-carbon (0.4 wt.% C) advanced high-strength steel processed via quench-and-partitioning (Q&P). Real-time diffraction data, captured during quench stop temperatures of 200 °C (QT200) and 240 °C (QT240), followed by a 1000 s holding time at the partitioning temperature of 300 °C, revealed precise phase fractions during the Q&P. However, the retained austenite in both process routes produced comparable retained-austenite fractions at room temperature —23% for QT200 and 21% for QT240—the higher quench-stop temperature generated three times more fresh martensite (15% vs. 5%). The mechanical properties were examined by tensile tests, showing that the lower fresh-martensite content in QT200 promotes progressive, strain-induced austenite transformation, delays necking, and yields a uniform elongation of 6.9%. By contrast, QT240 reaches a higher ultimate tensile strength (around 2023 MPa vs. 1984 MPa) and yield strength (about 983 MPa vs. 938 MPa) at the expense of ductility (around 4.7% uniform elongation). In both conditions, the TRIP effect is active, but its contribution is curtailed by the presence of fresh martensite. The present study thus establishes a quantitative link between in-situ phase evolution pathways and the resulting strength–ductility balance, providing guidance for tailoring Q&P processing route in medium-carbon advanced high-strength steel for crash-critical automotive applications.

Abstract: The present work was undertaken to understand the phase transformation behaviour in a third generation steel 0.22C-2.1Mn-1.0Si during continuous cooling. The microstructure at various cooling rates were examined by using different techniques, such as optical microscopy, scanning electron microscopy, dilatation test and X-ray measurement. The results show that the amount of bainite that forms during continuous cooling is limited and there is a bainitic transformation stop temperature for this kind of steels. A continuous cooling transformation diagram of the steel is established.

Abstract: It was investigated that the effects of retained austenite (γR) conditions on ductility of advanced high strength steels for automotives. 0.4mass% C steels were heattreated in various austemper conditions to control the retained austenite conditions. In the result of the evaluation of mechanical properties of these steels, it was confirmed there were steels which indicated different elongation even if they had almost same volume fraction and carbon content of retained austenite. In order to clarify the reason, the conditions of retained austenite and work hardening behavior were investigated in detail. It was indicated that the existence of high carbon content region in a part of retained austenite promoted the deformation induced martensitic transformation in the high strain range and improved the elongation of AHSS.

Abstract: Modern advanced high strength steels (AHSS) for the automotive sector often contain retained austenite which promotes remarkable combinations of strength and ductility. These high strength steels may however be subject to a risk of hydrogen embrittlement. For the current contribution, hydrogen trapping and embrittlement behaviour were investigated in AHSS compositions having different levels of retained austenite. Hydrogen permeation tests revealed that hydrogen diffusion was slower for increased levels of retained austenite, being controlled most likely by reversible trapping at austenite-matrix interfaces. External hydrogen embrittlement tests via step loading also revealed that resistance to hydrogen was lower for increased levels of retained austenite. It was suggested that during step loading the hydrogen accumulated at austenite-matrix interfaces, leading to cracking when the applied stress was high enough.

Abstract: Advanced High-strength steels (AHSS) has widely application in automotive due to their high tensile strength and remarkable ductility. These good mechanical performances are strongly influenced by the processing and final microstructure. This paper performed Deformation Dilatometer and finite element simulation to study the effect of hot rolling parameters such as strain, cooling rate, and holding time at constant temperature on the microstructure formation of Nb-V low carbon microalloyed steel grade. It found that increasing deformation degree increased the volume fraction of ferrite, both of deformation dilatometer and finite element simulation give a similar trend of effects of hot rolling parameters on evolution of volume fraction of ferrite. These results give an insight for industrial application.

Abstract: Despite that the conventional CSiMn TRIP steel has a promissing mechanical attributes, it has a limitations on the galvanizability of such grades of steel due to Silicon. Thus, aluminum as a strong candidate for substituting silicon has been introduced in this study accompanied by vanadium as a microalloying element. Microstructure of the studied steel was observed by using OM, and SEM. X-ray diffraction analysis, and tent-etching technique carried out on the studied steel to identify the fractions of the retained Austenite after thermal mechanical process, as well as its morphology. In addition, the mechanical properties in term of strength, ductility, strain hardening, and the rate of strain hardening were studied to define the influenced parameters throughout this alteration. The results refer to the possibility of complete replacement of silicon in TRIP steel with aluminum at the presence of vanadium as a micro alloying element.

Abstract: TRIP effect containing steel was well reputed by its high mechanical properties among the 1^st generation of Advanced High Strength Steel. High Silicon content was well established as an inhibitor for cementite precipitation at para-equilibrium condition. However, the effect of manganese as a powerful stabilizer for retained austenite was not much studied in TRIP-Steel. Thereby, the effect of high manganese content on the TRIP containing steel is studied in this research. As been observed from OM, and XRD results, it was found that as long as increasing Manganese content, the fraction of retained austenite increases. No doubt that enrichment of retained austenite throughout the matrix, beers a great impact on the plastic deformation character of the investigated steels, which was proved by using a uniaxial tensile test and determining the strain hardening exponent.

Abstract: Annealed Martensite Matrix (AMM) concept was studied on two steel grades with low alloyed base composition of Fe-C-Mn-Si and two levels of Nb. Conditions for the thermal treatments were selected based on the experimental dilatometry tests and thermodynamic calculations. Annealing trials with short austempering holding were performed in the laboratory salt pots. Mechanical properties of heat treated steels have been investigated by tensile tests. Associated microstructures have been analyzed using Scanning Electron Microscopy as well as magnetization saturation method for measuring retained austenite fractions. Excellent strength-ductility balance was obtained due to the ultra-fine multiphase structure and high amount of stable retained austenite.

Abstract: The simulation of industrial rolling processes has been shown to be an important method to optimize rolling parameters, reduce production costs and improve product quality. Previous works have shown the value of hot rolling simulation by means of torsion tests where the mean-flow-stress (MFS) can be successfully predicted. In the present work, three rolling schedules are simulated by hot torsion tests and compared. It is important to note this methodology provides the flexibility to test different ideas without the risk of downtime or damage to plant equipment that could result from an unsuccessful industrial trial. The simulation analysis considered the production steps from reheating through the final accelerated cooling as well as the final product microstructures. The study provides important information to the production of various steel grades such as pipeline, shipbuilding, structural and other high-end products.

Abstract: The addition of titanium is a well-known microalloying concept for hot rolled structural steels. Concerning advanced high strength steels for the automotive sector, the use of Ti microalloying (usually with Nb-V) has been an active research area. However, Ti addition has not been explored in depth. For the current contribution, a laboratory hot rolled 0.2C-2.4Mn-1.5Si steel with and without Ti addition was studied. Mechanical testing of the hot strip revealed a very high UTS (1GPa) for the Ti added steel, whilst for the unalloyed chemistry the UTS was some 300 MPa lower. Observation of the hot rolled microstructures via optical microscopy showed a significantly higher hardenability for the Ti added steel. Moreover, X-ray diffraction analysis indicated a significant amount of retained austenite in the Ti added strip, which transformed completely to martensite after the tensile test. Further analysis via TEM and chemical extraction indicated that Ti was present both as Ti (C,N) precipitates and in solution. Finally, in light of these observations, the possible mechanisms leading to the enhanced hardenability observed for the Ti added hot rolled strip steel were discussed.