Dynamic strain-induced transformation (DSIT) enables the formation of fine-grained ferritic microstructures, which are well suited for cold forming processes in the as-rolled condition. In this work, the formation mechanism, chemical composition, and crystallographic orientation of DSIT ferrite were investigated in a micro-alloyed steel and compared to pre-eutectoid ferrite. High-resolution techniques, such as scanning transmission electron microscopy and atom probe tomography (APT), were used for the investigations. To generate DSIT ferrite and pre-eutectoid ferrite, different experimental routes were applied using a compression deformation dilatometer. The results show a large number of NbC precipitates within DSIT ferrite, and show that the formation of DSIT ferrite is accompanied with C diffusion and the formation of retained austenite. APT measurements revealed that the C- and Mn concentration in DSIT ferrite is higher compared to pre-eutectoid ferrite. The crystallographic orientation of DSIT ferrite was examined using electron backscatter diffraction. The crystallographic orientation of DSIT ferrite after the deformation route revealed that the <111> plane normals are parallel to the compression direction with the <110> directions pointing towards the radial direction of the compressed sample. The results suggest that the formation of DSIT ferrite is a displacive mechanism, accompanied by C diffusion.

To achieve higher strength and good hardenability and at the same time use the positive effects of thermomechanical controlled processing, the concept of Nb and Ti microalloyed steels is increasingly used for high‐strength low‐alloy (HSLA) steels with higher C contents. Herein, how the addition of Ti affects the grain growth and static recrystallization behavior of a Nb microalloyed HSLA steel with a C content of 0.23 wt% is investigated. For this reason, alloys with varying Ti and constant Nb content are produced and investigated by means of annealing and double‐hit deformation experiments. Atom probe tomography measurements reveal that the Nb concentration in the matrix decreases with increasing Ti content. Therefore, the static recrystallization behavior is steadily inhibited with decreasing Ti content, as more dissolved Nb is available for the formation of strain‐induced NbC precipitates. The annealing experiments show that the combined addition of Ti and Nb is most effective against grain coarsening, regardless of whether the Ti content is 90 or 180 ppm. To use the positive properties of Ti against grain coarsening and Nb to inhibit recrystallization, a middle content must be chosen when alloying Ti to HSLA steels with higher C content.

Superduplex steels exhibit a microstructure of approximately equivalent fractions of austenite and δ-ferrite. This structure combines a higher strength than austenitic steels with a higher toughness than ferritic steels and an excellent corrosion resistance. Superduplex steels can be processed by different routes such as casting, extrusion, rolling or forging and are applied in the chemical industry, oil production or paper manufacturing. It is well known that the two phases exhibit a different dynamic restoration behavior due to their differences in the stacking fault energy. The austenite grains are more likely to undergo discontinuous dynamic recrystallization while the δ-ferrite grains tend to strong dynamic recovery. Modern large area electron back scatter diffraction (EBSD) scans are a powerful technique to study the microstructural evolution of the individual phases during hot-forming of duplex steels. However, detailed EBSD studies explaining the flow behavior, influence of grain orientation and grain size modification during hot forming have not been carried out yet. In the present investigation specimens of a S32750 superduplex steel were deformed in uniaxial compression with strain rates between 0.01 and 1 [s-1] to a true strain of 1 at temperatures between 1000 and 1300°C. The microstructures of the as-deformed specimens were examined by large area EBSD scans with particular attention to the characteristics of the individual phases as for example grain size and stored energy of the austenite grains or subgrain size and grain boundary character of the ferrite grains. The differences due to deformation temperature and strain rate are discussed.

Lean-Duplex-Stähle stellen eine kosZusammenfassung: tengünstige Alternative zu austenitischen und StandardDuplex-Stählen dar. Erreicht wird dies durch Verringerung des Nickel-und Molybdängehaltes sowie deren teilweise Substitution durch Mangan und Stickstoff. Dadurch werden auch die mechanischen Eigenschaften verbessert. In der Arbeit werden die beiden Lean-Duplex-Stähle X2CrMn-NiN22-5-2 (LDX 2101) und X2CrNiN23-4 (SAF 2304) hinsichtlich ihres mechanischen und chemischen Verhaltens, nach unterschiedlichen Auslagerungszeiten bei 475 °C und 700 °C, miteinander verglichen. Die dabei auftretenden Unterschiede sind auf Entmischungen (α'-Phase), Ausscheidungen (σ-Phase, Karbide und Nitride) und Bildung von Sekundäraustenit zurückzuführen. Mechanical and Chemical Behaviour of Lean-duplex Steels after Heat Treatment in Critical Temperature RangesLean-duplex steels represent a low-cost option Abstract: compared to austenitic and standard-duplex steels. This is achieved by reducing the content of nickel and molybdenum through substitution with manganese and nitrogen, respectively. These elements also improve the mechanical properties. In this work, two lean-duplex steels X2CrMn-NiN22-5-2 (LDX 2101) and X2CrNiN23-4 (SAF 2304) are compared with respect to their mechanical und chemical behaviour after different annealing conditions at 475 °C and 700 °C. All visible differences are the consequence of phase separation (α'-phase), precipitations (σ-phase, carbides and nitrides) as well as the formation of secondary austenite. Korrespondenzautor: OR Dipl.-Ing. Dr. mont. Michael Panzenböck, Department Metallkunde und Werkstoffprüfung, Montanuniversität Leoben,

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