Hot Ductility of Nb- and Ti-Bearing Microalloyed Steels and the Influence of Thermal History

Abstract

The hot ductility of Nb, Ti, and Nb-Ti containing steels has been studied under direct-cast conditions. A Gleeble 3500 thermomechanical simulator was used to determine hot ductility over the temperature range 1100°C to 700°C at a low strain rate of 7.5 9 10 À4 s À1 . Tensile samples were cooled at two different cooling rates, 100°C/min and 200°C/min, simulating, respectively, thick and thin slab casting processes. Complex thermal patterns designed to simulate the cooling conditions experienced near the surface of a slab during continuous casting were carried out for the Nb-Ti steel. The Nb-Ti steel had lower ductility than both the Nb and Ti steels. Increasing the cooling rate generally deteriorated ductility. The low recovery of ductility at higher temperatures is explained in terms of a low strain rate and fine precipitation delaying the onset of dynamic recrystallization. This can promote intergranular cracking as a result of grain boundary sliding in the austenite. At lower temperatures, ductility was further reduced due to the formation of thin ferrite films at the prior austenite grain boundaries. Simulating the thermal history experienced near the surface of thin (90 mm) cast slab improved ductility of the Nb-Ti steel by promoting coarser NbTi(C,N). This exposes a potential flaw in a simplified hot-ductility test: a failure to accurately represent the influence of the thermomechanical schedule on precipitation and, hence, hot ductility.

The ductility behavior of a high-Mn twinning induced plasticity steel during cold-to-hot deformation

“…The observed facet grain surfaces with shallow dimples are characteristics of failure due to GBS, i.e., intergranular decohesion. Similarly, in C-Mn-Nb steel, Carpenter et al [25] found that the fracture surfaces obtained from hot tensile specimens tested in the single phase austenite region exhibited intergranular decohesion, displaying facet, due to the GBS.…”

Hot Ductility Behavior of a Peritectic Steel during Continuous Casting

“…Specimens are then heated slowly up to the temperature which melting occur and were held at this temperature for 60 s to heat the middle zone of specimens homogenously. A small amount of compressive deformation (2% to 10%) [7,20,21] is axially applied onto the specimens during first solidification step through to 1380 °C to be able to eliminate the shrinkage. Approximately 2% compressive strain was applied to the specimens in these tests.…”

The Influence of Warm Rolling Reduction on Microstructure Evolution, Tensile Deformation Mechanism and Mechanical Properties of an Fe-30Mn-4Si-2Al TRIP/TWIP Steel

“…The fracture surface of the as-annealed specimen is the typical feature of ductile fracture by the presence of isometric dimples (Figure 5a). Furthermore, there are some flat regions marked with white arrows in Figure 5d, indicating failure due to grain boundary sliding in the 55% condition [32]. The corresponding dimple diameter distribution of samples at different rolling reductions is depicted in Figure 5e.…”