Effect of Thermomechanical Processing on the Hot Ductility of a Nb-Ti Microalloyed Steel.

Abstract

Many attempts have been made to understand the problem of transverse cracking in continuous casting process. Much of this research has involved the study of hot ductility using 'conventional' isothermal hot ductility testing. In these tests, the specimens were isothermally tensile tested to fracture, at temperatures achieved by cooling from a solutionizing temperature, close to the solidus, or above the liquidus. These studies showed that hot ductility at the test temperature is highly depended on the thermal path followed by the specimens. The thermal histories experienced by strands during continuous casting were found to be quite complex and invariably involve rapid cooling and heating cycles. This may therefore lead to high thermal gradients, which, in turn, can generate strains in the surface of the solidifying strand. This then may alter the microstructural evolution of the strand surface and the corresponding hot ductility, a possibility that has not been addressed in any previous studies. Thus, the purpose of this study was to consider the effect of the thermomechanical history on the hot ductility of steel.After in-situ melting and solidification, Nb-Ti microalloyed tensile specimens were subjected to a thermal history typical of a continuously cast billet. Different degrees of deformation were imposed on the specimens at selected stages of this thermal history, before tensile testing to fracture point at the time and temperature corresponding to the unbending stage of the billet casting. It was found that the hot ductility varied from 1 to 98 %, depending on the stage in the thermal history at which deformation was executed. The microstructural evolution during the thermomechanical profile was followed to study the effect of thermomechanical history on the hot ductility.

Grain Refinement of As Cast Austenite by Dynamic Recrystallization in HSLA Steels

“…Currently S. Yue et al conducted hot ductility study of the Nb-Ti steel by using similar experimental procedures to the present case, where it was reported that g grain size obtained after melting and solidification was around 120 mm, although it was not referred about it's coincidence to actually observed g grain size in the solidified steel billet. 23) G. E Ruddle et al studied direct rolling of thin slabs of the Ti-Nb steel by Gleeble simulation of continuous casting, and reported that g grain size after solidification was varied from 525 to 148 mm as the cooling rate increased from 0.5 to 15 K/s. 6) Comparing with these results, g grain size obtained by the present melting and solidification experiment was much larger and corresponded more closely to g grain size actually observed in the solidified strand cast slab with thickness over 200 mm.…”

Ultrahigh Strength Steel: Development of Mechanical Properties Through Controlled Cooling

“…It normally increases the tensile properties and toughness (Dhua, 2003) without reducing ductility or brittle fracture resistance (Akhlaghi, 2001;Jahazi & Egbali, 2000). With controlled rolling it is possible to refine the ferrite structures directly after finish rolling or by using additional accelerated cooling.…”

Strip Casting of a High‐Manganese Steel (FeMn22C0.6) Compared with a Process Chain Consisting of Ingot Casting and Hot Forming

“…As a result, casting and hot rolling have to be performed within a very narrow process window in order to avoid cracking. This is described in the literature on these steels in papers dealing with direct strip casting 9, high temperature strength and crack susceptibility 10, thermomechanical rolling 11, and the effect of the reduction ratio during the first pass in hot rolling 8.…”