Combined Macro–Micro Modeling for Rolling Force and Microstructure Evolution to Produce Fine Grain Hot Strip in Tandem Hot Strip Rolling

Abstract

Micro-scale Analysis of Microstructure Evolution and Rolling Force Figure 15 illustrates the macro-micro combined model for rolling force and microstructure evolution, and analytical elements that constitute the model. 23) In the rolling process, various changes occur concurrently; viz. plastic deformation at the roll bite, microstructure changes as a result of recrystallization and transformation both during and after rolling, and temperature changes due to plastic heat and cooling by processing. Moreover, as tandem hot strip rolling is a multi-forming process, these changes are repeated a number of times. To analyze with high accuracy a complex field in which deformation, microstructure changes, and temperature changes interact with each other, we built a macro-micro combined model.Figures 16(a) and 16(b) verify the estimation accuracy of rolling load by the combined macro-micro model. Four results are shown here: measurements, estimation by the conventional macro-micro combined model, estimation by the macro-micro combined model with asymmetric rolling and adjustable material data and estimation by the new macro-micro combined model with the dynamic ferrite transformation using the intragranular nucleation model. In low reduction rolling, Fig. 16(a) shows the conventional macro-micro model is proven capable of estimating rolling force to a high degree of accuracy with consideration for asymmetric rolling and adjustable material data. But, in high reduction rolling, however, the estimation accuracy of the conventional macro-micro model was low even when considering the asymmetric rolling theory for the last three stands and introducing adjustable material data. At 50 % or less the relative estimation accuracy of F6 stand, the model could not formulate metallurgical phenomena well. The new macro-micro combined model with dynamic ferrite transformation using the intragranular nucleation model improved the prediction accuracy of rolling force in F6 stand.Figures 17(a) and 17(b) show the estimation accuracy of rolling load at the F6 stand by the conventional macromicro combined model and by the new macro-micro combined model respectively. Conventional strips were produced by the conventional schedule rolling in low reduction and high temperature at the last three stands. Fine grain strips were produced by the new schedule rolling in high reduction and low temperature at the last three stands. These graphs indicated that rolling load was estimated well by the new model. Figures 18(a) and 18(b) show the estimation accuracy of ferrite grain size by the conventional macromicro combined model and by the new macro-micro combined model. These graphs indicated that ferrite grain size could be estimated in both conventional strips and fine grain strips. Figure 19 presents the results of analysis by the new macro-micro combined model, made to see which nucleation, intergranular or intragranular, acted dominantly over the other. Conventional schedule rolling was low reduction and high finishing temperature rolling in the las...

Grain Refinement in Dual-Phase Steels

“…This could be considered in new mill designs, but will first require a detailed assessment of the robustness of the proposed processing path. Alternatively, recent work by Morimoto et al [37] suggests that DIFT can be initiated in the final stands of a seven-stand finish mill by proper interstand cooling in conjunction with asymmetric rolling that produces significant shear strain to reach the critical strain levels for UFF formation. In any event, a distinct advantage of the DIFT route would be that ferrite forms very rapidly within less than 1 second, thereby eliminating the need for a slow cooling period of more than 10 seconds through the ferrite formation stage that is usually employed in the current production of hot-rolled DP steels.…”

Grain refinement and mechanical properties of asymmetrically rolled low carbon steel

“…This additional shear strain could provide a shear texture in rolled strips (Sidor et al, 2008) and was reported to refine the ferrite grain size providing a larger plastic strain compared to the symmetric rolling in a lab scale hot rolling (Salvatori et al, 2008) and in a industrial tandem hot rolling (Morimoto et al, 2007). Another advantage of the rolling process in the production of fine ferrite grains is the ease of the control of cooling rate after rolling.…”

Phenomenological Model for Deformation-Induced Ferrite Transformation

“…[5,6] Initially, it was thought that a coarse austenite microstructure (austenite grain size of the order 200 lm) in combination with surface chilling by contact with the rolls and the significantly higher deformation degree in the surface region leads to the required conditions for DIFT. Recently, Morimoto et al [9] reported industrial production of a fine-grained plain-carbon steel that is attributed to DIFT, which was initiated in the final stands of a seven-stand finish mill by a combination of asymmetric rolling and proper interstand cooling.…”