Understanding the high temperature side of the hot ductility curve for steels

Abstract

The study tests the validity of the tensile hot ductility test for assessing cracking during the straightening operation. Steels with a thin film of deformation induced ferrite (DIF) or fully austenitic when straightening were examined. In both cases dynamic recrystallisation (DRX) occurs at high temperatures. However, DRX is not possible on straightening, the grain size being too coarse and strains too low. When, DRX occurs, ductility is overestimated compared to the un-recrystallised condition on bending. For steels with DIF films if the depth of the trough is ≥ 40%RA (reduction of area) cracking is unlikely. However, for TWIP steels, the estimated RA for unrecrystallised ϒ can be much < 40% causing cracking even though measured ductility is well in excess.

Assessing the Effects of Steel Composition on Surface Cracks in Continuous Casting with Solidification Simulations and Phenomenological Quality Criteria for Quality Prediction Applications

“…The decomposition reaction of austenite is also an important factor contributing to the risk of transverse cracking during the straightening of the strand, as the ferrite film suffers from decreased ductility until the volume fraction of ferrite increases to around 40%. [ 44 ]…”

“…The decomposition reaction of austenite is also an important factor contributing to the risk of transverse cracking during the straightening of the strand, as the ferrite film suffers from decreased ductility until the volume fraction of ferrite increases to around 40%. [44] In addition to longitudinal and transverse cracking, numerous other defects such as slag defects can form during continuous casting. These defects may originate from the infiltration of either mold or tundish slag in the liquid during continuous casting and from reoxidation products.…”

Hot Ductility Evolution Mechanism of Titanium‐Bearing Microalloyed Steels

“…This is because the deformed austenite of the tensile sample at 950 and 1100 °C can produce beneficial dynamic recrystallization (DRX), the DRX can trigger grain boundary migration away from cracks, reduce grain boundary sliding, and limit the development of cracks caused by austenite grain boundary slip and microcracks near precipitate particles within grains. The development of the crack can only cross the whole grain through the shear stress at the crack tips, [33][34][35][36] so the fracture mechanism of the tensile sample over the range of 890-1350 °C is ductile transgranular fracture, and the greater the difficulty of crack propagation in the sample, the better the plasticity of the sample. Besides, shown in Figure 11a3, b3, the microstructure near the fracture of the tensile samples at 950 and 1100 °C is martensite single-phase region which was transformed from single-phase austenite region due to the rapid cooling treatment by Gleeble-3500 thermal mechanical simulator.…”

The influence of grain size and precipitation and a boron addition on the hot ductility of a high Al, V containing TWIP steels

“…However, DRX does not occur in the base of the trough in these low C steels, Figure 1, and therefore if the RA value is 35-40% at the base, it can be taken that the steel will not show cracking during straightening at higher temperatures when the steel is fully austenitic, and the ferrite film is no longer there. Even if DRX occurs in the tensile test inflating the ductility, the absence of the ferrite film will enhance ductility sufficiently to avoid cracking on straightening [1].…”

“…Nevertheless, for austenitic steels, it is difficult from the tensile test to separate the ductility due to DRX, from that occurring in the un-recrystallised austenite, the latter being what is important for preventing cracks from forming during the straightening operation [1]. This problem has been found particularly so for the austenitic high Al, high Mn, TWIP steels where the ductility of un-recrystallised austenite actually decreases with increasing test temperature and may be well below the benchmark requirement of 35-40% to avoid cracking in the straightening operation temperature range, 700-1000 °C [1].…”