Influence of V and Ti on hot ductility of Nb containing steels of peritectic C contents

Abstract

The hot ductility of in situ melted tensile specimens of Ti–Nb containing steels having C contents in the peritectic C range 0·12–0·17 with and without V has been examined over the temperature range 700–1000°C. An improved testing regime for simulating the continuous casting process was used, which takes into account both primary and secondary cooling conditions. For the Nb containing steels, the ductility improved in the temperature range 750–850°C as the Ti/N ratio increased. However, ductility at 800°C was still below the 35–40 reduction in area values required to avoid transverse cracking. This was attributed to the copious precipitation of sub 40 nm NbTi(CN) precipitates along the grain boundaries and finer precipitates within the grains. Adding V to the Ti–Nb containing steels resulted in significantly improved ductility with reduction in area values at 800°C in excess of 45. This improvement was due to a decrease in the fraction of fine particles, and in accord with this better ductility, transverse cracking of industrial slabs was avoided.

Comparing the Hot Ductility Behaviour of Low‐Carbon Microalloyed Nb‐V‐Ti Steels During Two Thermal Cycling Routes: Solutionizing and Precipitation Treatments

“…In addition to this expectation, the prescribed holding time of 240 s at high temperatures before testing may also encourage the coarsening of any precipitates (e.g., TiN) at the expense of other ones (Nb‐V(C,N) that become ineffective in pinning grain boundaries or causing further cavity linkage . Thus, whatever the case, the resulted fracture may occur in a ductile manner, (Figure a and b) . However, it should be noted that the conditions imposed on CS steel during tensile testing (in DTC) may differ significantly from those experienced by the product during straightening in continuous casting.…”

“…However, in the two‐phase region, one must think that stress concentrations at the grain boundary‐AlN inclusion interface seem to be a secondary reason for the ductility loss. Such an expectation builds on the amount of AlN precipitates which is closely related to the [Al][N] solubility product, as stated by Banks et al These authors put forward a trivial effect of AlN precipitates when this product is less than 2.5 × 10 −4 (i.e., 0.05% Al and 0.005% N), which is consistent with that calculated equal to 10 −4 in our case (i.e., 0.017% Al and 0.006 % N).…”

“…Zarandi and Yue as well as Kim et al showed that a substantial improvement in ductility could be achieved through dynamic recrystallization, producing fine‐grained structure and preventing void formation and coalescence. El‐Banks et al considered the cooling conditions of the slab, pointing out a substantial improvement in ductility when a gradual cooling replaces conventional air‐cooling . Lee et al found a reasonable improvement in ductility of Nb‐Ti steels in the two‐phase domain through simultaneous thermal treatments …”

Control of Transverse Corner Cracks on Low-Carbon Steel Slabs

“…[6][7][8][9][10][11][12][13] Furthermore, the secondary cooling is directly linked with the formation of transverse corner cracks. 14 At the bending and straightening segments during the continuous casting process, an appropriate cooling method to ensure the temperature of the slab corner to avoid the poor hot ductility temperature range is essential to avoid transverse corner cracks.…”

A Method to Control the Transverse Corner Cracks on a Continuous Casting Slab by Combining Microstructure Analysis with Numerical Simulation of the Slab Temperature Field

“…Most of the fracture slab surfaces indicate a creep‐type failure due to the coalescence of cavities nucleating at the grain boundaries, including nitrides (AlN, Nb(C,N), BN, etc. ), proeutectoid ferrite film, and cementite …”