Daniel Krizan

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.

Investigation on the Microstructural Evolution in a Medium-Mn steel (X10Mn5) after Intercritical Annealing*

Abstract TRansformation Induced Plasticity (TRIP)-assisted Medium-Mn steels, with Mn-contents in the range of 4–10 ma.-%, have recently gained a lot of interest due to their promising mechanical properties. This steel group contains ≥ 30 % of retained austenite, which is stabilized by Carbon and mainly Manganese during intercritical annealing. The present work investigated the influence of annealing temperature and cooling rate on the microstructural evolution by means of dilatometry. Two thermodynamical models for the prediction of optimal annealing temperature and maximum retained austenite content have also been thoroughly evaluated. For further characterization, scanning electron microscopy, EBSD, micro-hardness testing and X-ray diffraction were carried out. The investigations manifested a pronounced influence of both annealing temperature and cooling rate, on the phase fractions of ferrite, austenite and martensite, which must be taken into account by design of batch annealing route for Medium-Mn TRIP steels in order to obtain superior combination of strength and ductility.

The Effects of Intercritical Annealing Temperature and Initial Microstructure on the Stability of Retained Austenite in a 0.1C-6Mn Steel

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.

Hot Deformation Behavior of a 3rd Generation Advanced High Strength Steel with a Medium-Mn Content

Medium-Mn steels are one of the promising candidates to achieve the desired mechanical properties in the 3rd generation of cold rolled advanced high strength steels (AHSS) for automotive applications. Their duplex microstructure consists of a ferritic matrix with a substantial amount of metastable retained austenite, which transforms to strain-induced martensite upon forming. This strengthening mechanism, well known as the TRansformation Induced Plasticity (TRIP) effect, provides the steel an excellent combination of high strength and elongation with a product of RmxA80 up to 30.000 MPa%. As hot rolling is one of the crucial steps during their production, the hot deformation behavior of Medium-Mn steels has to be thoroughly evaluated during their development stage.Therefore, the present contribution studied the hot deformation response of a 0.1 %C 5.5 %Mn steel by means of hot compression tests using a Gleeble® 3800 device. The influence of different deformation temperatures (900-1100 °C) and strain rates (0.1-10 s-1) on the stress-strain behavior was investigated. The flow curves were analyzed and corrected by the effects of adiabatic heating.Furthermore, the strain rate sensitivity m of the material was determined by evaluating stress values at different strain rates for given temperatures and strains. The m-values can be used to predict the deformation behavior of the material within the investigated range of parameters.Lastly, the hot working behavior of an alternative steel concept for a 3rd Generation AHSS with significantly lower Mn-content was comparatively investigated.