The hot ductility and malleability of a vanadium-microalloyed steel is investigated by means of tensile and compression tests at temperatures ranging from 700 to 8508C and strain rates of 3 Â 10 À4 to 0.3 s À1 . The deformation tests are performed after austenitization and cooling to test temperature. The so-called second ductility minimum is located around 7508C for all strain rates except for the highest one, where no ductility trough is observed. Ductility steadily increases with strain rate at a given temperature, and the fracture mode progressively changes from intergranular to transgranular. In the region of minimum ductility, intergranular cracking occurs at low strain rates by void nucleation, growth and coalescence within thin layers of deformation induced ferrite covering the austenite grain boundaries. Cracking is favoured by V(C,N) precipitation associated with the g/a phase transformation. Ductility remains low above the temperature of minimum ductility, where no apparent ferrite formation is observed (790 8C). Void formation takes place as a result of grain boundary sliding in combination with matrix and grain boundary precipitation. These voids are able to grow and link up forming intergranular cracks. Ductility increases with strain rate mainly due to the short time available for precipitation as well as for intergranular void growth and coalescence.

4Voestalpine, Linz, Austria temperature tensile tests, and efforts are still needed to understand the crack formation mechanisms [4]. However, the internal stress situation which may lead to damage initiation and propagation during continuous casting is still unknown. The current work describes an experimental approach by non-destructive testing to determine the evolution of internal stresses under simulated production conditions. ExperimentalAngle dispersive neutron diffraction [5] was carried out for a 4.8 wt.% Ni containing low carbon steel alloy during cooling from casting. Samples of 2 × 5 x 15 cm 3 were cut from the surface of the slab representative for the cross section affected by heterogeneous cooling. An induction furnace was mounted on the sample table of the strain scanner to induce a thermal gradient (Fig. 1a). The coil was set on top of the sample and an actively cooled by a Cu heat sink from the bottom. After a stable mean thermal gradient of ~ 10 K/mm (Fig. 1b) was established, the induction coil was deactivated and the sample cooled from the bottom. Each scanned point was first heated to 1300 °C (100 % austenitic condition) and held for 15 min before establishing the thermal gradient. A monochromatic neutron beam was continuously diffracted (1 min/scan) in the center of the steel sample within a gauge volume of 5 × 5 x 5 mm³ penetrating 2 cm in the steel sample. The temperature dependent phase strain values in austenite (311) and ferrite (211) at several depth positions (4, 6 and 8 cm from the bottom) were determined relatively to a stress free reference sample (cylindrical, d ~ 3 mm, l ~ 10 mm) < gauge volume without dendritic morphology extracted from the surface of the slab. Abstract:In the steel industry, steel slabs are produced by continuous casting to allow fast processing. A complex stress state leads to crack formation in such steel slabs. The stress components relevant for failure were identified by neutron diffraction under simulated casting conditions. Eigenspannungen in StranggussbrammenZusammenfassung: Stahl wird industriell mittels eines kontinuierlichen Gussprozesses erzeugt, um rasche Produktion zu gewähren. In Stranggussbrammen führen Eigenspannungen zu Rissbildung, welche mittels Neutronenstreuung, zerstörungsfrei, unter simulierten Gussbedingungen identifiziert werden konnten.

Hot tensile tests are performed on a Ni-alloyed steel after solution treating and cooling to test temperatures ranging between 600 and 950°C at strain rates of 3∙10-4and 3∙10-3/s. The critical reduction of area (RA) for damage formation is determined by means of laboratory X-ray computed tomography. The results are compared with the corresponding RA at fracture traditionally used for assessing hot ductility and crack sensitivity during straightening of the slab. We demonstrate that using RA at fracture is inadequate for evaluating hot ductility in the austenite phase region at temperatures varying with the strain rate. Hot ductility curves more relevant for slow strain rate deformation are determined.

Continuous casting is an established production route for steel slabs. The solidifying slab suffers a combination of internal stresses caused by thermal gradients, phase transformation, and external bending. The complex superimposed stress condition may produce local concentrations of tensile stresses that initiate cracks during cooling and deformation. Samples prepared from low-carbon steel slabs with and without Ni-content are compared and transformation stress phenomena are analyzed. In situ neutron diffraction experiments are carried out to reveal the stress condition within sections of cast steel slabs during cooling while passing the austenite-ferrite transformation temperature. The austenite-ferrite phase transformation is accompanied by an increase of volume of %1% between austenite-f.c.c. and ferrite-b.c.c., which causes internal stresses within the thermal stress gradients by cooling from the surface. The superposition of these thermal and transformation stresses is determined during cooling by an additional in situ diffraction experiment at constant macroscopic strain. The crack sensitivity of the Ni-containing slab is attributed to higher internal stresses within the austenite caused by the lower transition temperature than for the Ni-free steel.

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