Analysis of the Microstructure Development of Nb-Microalloyed Steel during Rolling on a Heavy-Section Mill

Abstract

It is not realistic to optimize the roll pass design of profile rolling mills, which typically roll hundreds of profiles, using physical modelling or operational rolling. The use of reliable models of microstructure evolution is preferable here. Based on the mathematical equations describing the microstructure evolution during hot rolling, a modified microstructure evolution model was presented that better accounts for the influence of strain-induced precipitation (SIP) on the kinetics of static recrystallization. The time required for half of the structure to soften, t0.5, by static recrystallization was calculated separately for both situations in which strain-induced precipitation occurred or did not occur. On this basis, the resulting model was more sensitive to the description of grain coarsening in the high-rolling-temperature region, which is a consequence of the rapid progress of static recrystallization and the larger interpass times during rolling on cross-country and continuous mills. The modified model was verified using a plain strain compression test (PSCT) simulation of rolling a 100-mm-diameter round bar performed on the Hydrawedge II hot deformation simulator (HDS-20). Four variants of simulations were performed, differing in the rolling temperature in the last four passes. For comparison with the outputs of the modified model, an analysis of the austenite grain size after rolling was performed using optical metallography. For indirect comparison with the model outputs, the SIP initiation time was determined based on the NbX precipitate size distribution obtained by TEM. Using the PSCT and the outputs from the modified microstructure evolution model, it was found that during conventional rolling, strain-induced precipitation occurs after the last pass and thus does not affect the austenite grain size. By lowering the rolling temperature, it was possible to reduce the grain size by up to 56 μm, while increasing the mean flow stress by a maximum of 74%. The resulting grain size for all four modes was consistent with the operating results.

Effect of Nb-Ti Microalloyed Steel Precipitation Behavior on Hot Rolling Strip Shape and FEM Simulation

“…The primary concern in the process of precision rolling involves the phenomenon of strain-induced precipitation, and many scholars have studied it [ 15 , 16 , 17 ]. Dutta and Sellars [ 18 ] were the first to propose a precipitation model for Nb microalloyed steels, incorporating the influences of material deformation and recrystallization factors.…”

Analysis of Recrystallization Kinetics Concerning the Experimental, Computational, and Empirical Evaluation of Critical Temperatures for Static Recrystallization in Nb, Ti, and V Microalloyed Steels

“…Hot rolling processes at elevated temperatures can induce coarse and non-uniform austenite grain structures, which impair the mechanical properties of the final steel product [1,2]. Additions of microalloying elements, e.g., titanium, niobium, and vanadium, to low-carbon steel help to prevent austenite grain coarsening due to pinning of grain boundaries by precipitates and solute drag [3][4][5].…”

Hot Deformation and Microstructure Evolution of Metallic Materials

“…The developed mathematical models were verified by a semi-industrial hot-rolling process. Sauer et al [ 12 ] analyzed the microstructure development of Nb-microalloyed steel during rolling on a heavy-section mill. A modified microstructure evolution model was presented that better accounted for the influence of strain-induced precipitation (SIP) on the kinetics of static recrystallization.…”