Optimization of Rolling Conditions in Nb Microalloyed Steel Processed by Thin Slab Casting and Direct Rolling Route: Processing Maps

Abstract

The microstructural evolution during the hot rolling of coarse grain sized austenite has been modeled taking into consideration all the microstructural mechanisms (dynamic, static and metadynamic recrystallization, strain induced precipitation) that could take place during the industrial TSDR production of a Nb microalloyed steel. Based on the results obtained from the model, processing maps have been drawn for a 0.035%Nb microalloyed steel. Optimum processing conditions to exploit all the benefits of Nb microalloying have been defined considering a final gauge thickness range between 1.5 and 12.65 mm.

Model Fe-Al Steel with Exceptional Resistance to High Temperature Coarsening. Part I: Coarsening Mechanism and Particle Pinning Effects

“…When the driving force P is less than the particle drag pressure P ZC shown in Eq. [11], ferrite grain growth will stagnate due to the complete pinning of grain growth by the austenite particles. If the driving force for grain growth is comparable to the pinning pressure, then some growth can take place due to the coarsening of the pinning particles at high temperature.…”

“…It can be shown that when the cross-diffusion effects are negligible, the coarsening coefficient predicted by Eq. [4] simplifies to 1…”

“…The same expression was applied by Martin et al [22] and Lee et al [26,27] In order to assess the applicability of this simplification to the present alloy system, the coarsening constant K, was evaluated from Eq. [4] using the full diffusion matrix and using Eq. [5], which only includes the diagonal diffusion coefficient.…”

“…Studies of TSCDR show that thermomechanical processing can reduce the average grain size, but cannot eliminate the large grains present at the center of the slab, especially for heavy gage products. [3,4] This makes it extremely important to control the as-cast microstructure and the grain size evolution at high temperatures in order to achieve acceptable final mechanical properties. Second phase particles have been used extensively to control the kinetics of grain growth at high temperature during TSCDR.…”

“…In the absence of particle pinning, austenite grains in excess of 2 mm may form prior to the onset of thermomechanical processing. [10,11] This work proposes a novel alternative, in which, a two-phase mixture of austenite and delta-ferrite is utilized to control the grain size evolution at high temperature. The basic concept is to design steels in which austenite and delta-ferrite co-exist over a wide temperature range.…”