Ludovic Samek

Mechanical Properties of Low Alloy Intercritically Annealed Cold Rolled Trip Sheet Steel Containing Retained Austenite

Abstract Key factors influencing the mechanical properties of low alloy intercritically annealed sheet steel containing high C retained austenite are discussed. Ferrous microstructures containing retained austenite are fundamentally different from most current automotive sheet steel products both in terms of their asproduced properties and application related performance (formability, bake hardening and high strain rate deformation). The properties are mainly determined by the C distribution and the latter is influenced by the composition, in particular the Si, Al and P contents, the main processing parameters (the intercritical and isothermal bainitic transformation temperatures and times) and the type of deformation. The properties of the retained austenite islands control the static strain hardening, the bake hardening and the high strain rate dependence of the transformation induced plasticity (TRIP) effect. The short term technical solution of fundamental questions related to low alloy intercritically annealed sheet steel containing high C retained austenite which are the influence of the distribution of substitutional solutes, the C distribution and the strain induced transformation of these steels are reviewed On discute des facteurs-clés influençant les propriétés mécaniques de tôle d’acier faiblement allié, à recuit intercritique, contenant de l’austénite résiduelle à haute teneur en carbone. Les microstructures de fer contenant de l’austénite résiduelle sont fondamentalement différentes des produits les plus communs de tôle d’acier pour automobiles, tant en termes de leurs propriétés à l’état brut que du rendement relié à l’application, i.e. formabilité, durcissement par cuisson et vitesse de déformation élevée. Les propriétés sont déterminées principalement par la distribution de C et cette dernière est influencée par la composition, en particulier la teneur en Si, Al et P, par les principaux paramètres de traitement, i.e. par les températures et les durées de transformation intercritique et bainitique isotherme ainsi que par le type de déformation. Les propriétés des îles d’austénite résiduelle contrôlent l’écrouissage statique, le durcissement par cuisson et la dépendance de la vitesse de déformation élevée de l’effet TRIP. On examine la solution technique à court terme des questions fondamentales reliées à la tôle d’acier faiblement allié à recuit intercritique, contenant de l’austénite résiduelle à haute teneur en C, i.e. l’influence de la distribution des solutés de substitution, la distribution du C et la transformation induite par la déformation de ces aciers.

Extended Tensile Ductility of a Formable High-Performance High-Manganese Steel

ZusammenfassungDas Ziel dieser Publikation ist eine Übersicht über den Einfluss der Struktur auf die Eigenschaften eines Hochleistungs-FeMnN Stahls zu geben. Im Rahmen dieser Arbeit wurde der Einfluss vom Mn-Gehalt von 12 Gew.-% bis 24 Gew.-% basierend auf dem Fe-C-Phasendiagramm durch thermodynamische Simulationen berechnet. Der Einfluss der Temperatur beim Haspeln auf die Mikrostruktur des Stahls wurde durch FeMnN Glühbehandlungen von 350 °C bis 750 °C untersucht. So konnte das mögliche Auftreten von Zementit und Martensit / Perlit nach langen Glühbehandlungen für Temperaturen nahe 500 °C nachgewiesen werden. Der untersuchte FeMnN Stahl bietet eine hervorragende Kombination von Zugfestigkeit und Duktilität an. Dieser FeMnN Stahl hat eine bemerkenswert hohe Bruchdehnung von etwa 100 % mit einer hohen Zugfestigkeit von 1100 MPa bei Raumtemperatur. Das Produkt von Bruchdehnung und Zugfestigkeit erreicht, proportional zur Zähigkeit, einen außergewöhnlichen Wert von etwa 110000 % MPa. Die erweiterte Zug-Duktilität des Hochleistungs-FeMnN Stahl wird der Nano-Zwillingsbildung induzierten Plastizität (NS-TWIP) zugeschrieben, die durch Transmissionselektronenmikroskopie nachgewiesen wurde.SummaryThe aim of the contribution is to review the structure/property relationship of a high-performance FeMnN steel composition. In this work the influence of the Mn content ranging from 12 wt.-% to 24 wt.-% on the Fe-C phase diagram was calculated by thermodynamic simulations. The influence of coiling temperature on the microstructure of the FeMnN steel was studied by annealing treatments from 350 °C up to 750 °C. It showed the possible presence of cementite and martensite/pearlite after long annealing treatments at temperatures close to 500 °C. The steel was observed to exhibit outstanding combinations of tensile strength and ductility. The FeMnN steel exhibited at room temperature a remarkably high total elongation of about 100 % with a high tensile strength level of 1100 MPa. This product, proportional to toughness, achieved an outstanding value of about 110000 %MPa. The extended tensile ductility of the high-performance FeMnN steel was attributed to the nano-size twinning induced plasticity (NS-TWIP), which was observed by in-depth transmission electron microscopy study.

Neutron diffraction analysis of martensite ageing in high-carbon FeCMnSi steel

Abstract A high-carbon metastable austenite phase was quenched in liquid nitrogen to obtain fresh athermal martensite. Different ageing treatments were performed in order to study the transformation of the martensite phase and the formation of carbides. Neutron diffraction experiments give detailed information on the transformations during the annealing treatment itself. The tetragonal athermal martensite transformed immediately, i.e. during the heating to the ageing temperature, to cubic martensite. Ageing at 400°C resulted in the decomposition of the austenite in ferrite and carbides and the formation of bainite. Those carbides could be determined as -carbides transforming in -carbides after extended annealing times. As was expected, ageing at 170°C resulted in the formation of a small amount of carbides while the austenite phase remained stable.

The Effect of Niobium on the Microstructure and Phase Transformation Kinetics in Low-Carbon Medium Manganese Steels*

Abstract Medium-Mn steels are a promising candidate to fulfil the requirements of the 3rd generation of AHSS (Advanced High Strength Steels) which combine high strength and high ductility. A new approach is to add micro alloying elements, like niobium, to affect microstructural development during recrystallization and phase transformations. Using a quenching dilatometer, the kinetics of the phase transformations from austenite (810 °C and 840 °C) to its transformation-products by varying different cooling rates (100 K/s to 0.03 K/s) were investigated for two low carbon medium-manganese steels with different contents of niobium. Based on the dilatometric curves, the results of light optical microscopy (LOM) and quantitative metallography, measurements of the retained austenite (RA) content by X-ray diffractometry (XRD) and hardness measurements, continuous-cooling-transformation (CCT) diagrams were constructed. Furthermore, the influence of the element niobium on the microstructure after intercritical annealing was investigated by similar methods. Different etching-methods (Nital, Klemm-I) were applied to reveal different microstructural aspects. Besides considerable grain-refinement effects with increased niobium content, the phase-transformations were accelerated in the CCT-diagrams. The microstructure varies from 100 % martensite to certain amounts of bainite, but also ferrite at lower cooling rates. Intercritical annealing results in different amounts of retained austenite.