The influence of rough rolling conditions on austenite evolution in peritectic microalloyed plate steels

Abstract

The thermo-mechanical processing of microalloyed steels has been studied widely. However most of the work has been focused on the finishing stages of hot rolling where pancaking of the austenite is desired for strength and toughness. During roughing, full recrystallization is desired for a uniform, through-thickness austenite microstructure. In this work the influence of the strain sequence during roughing on the evolution of austenite in plain carbon, C-Mn-V, C-Mn-Nb-V and C-Mn-Nb-Ti-V steels, was investigated during conventional hot rolling. This was done because strain is one of the most controlling parameters of recrystallization during hot rolling which in turn controls the austenite microstructure. Reheating and roughing simulations were conducted in a B�hr 805D deformation dilatometer using a constant austenitising temperature of 1220 �C. Different heating cycles were adopted in the reheating simulations to see the influence of heating rate and soaking time on the austenite grain size. Heating rate of 7.5 �C/min and constant soaking time of 30 minutes were used for roughing simulations as this closely simulated the slab�s centre thermal profile. The roughing strain sequence was varied from small (0.07 _/ pass), intermediate (0.1 _ / pass) to large (0.15 _ / pass) at a temperature range of 1150-1050 �C and strain rates of 0.3-2.9 s-1. Application of a non-standard 0.4 strain in the last pass of each strain schedule was introduced to study its effect on the austenite grain size. Double stroke tests were used to quantify the austenite softening behaviour. The prior austenite grain size was measured from quenched specimens. A FEM study was commissioned to study the localized strain distribution in axisymmetric compression and flat plate rolling simulations. Reheat austenite grain sizes increased with increasing soaking time and decreasing heating rate. The C-Mn-Nb-Ti-V steel had the finest reheat grain size and the C-Mn steel was the coarsest. This was attributed to the presence of the stable TiN in the former. The austenite grain size after roughing simulations decreased with increasing strain per pass. The C-Mn had the coarsest austenite grain size after roughing followed by the C-Mn-Nb-V and C-Mn-Nb-Ti-V the finest. However application of the 0.4 strain in the last pass led to similar grain sizes across the steels especially at high exit temperatures. At strains of 0.1 the Nb-containing microalloyed steels had a lower volume recrystallized fraction magnitude in the partially recrystallized passes compared to the plain C-Mn and the C-Mn-V steels. This is considered to be due to the retardation of recrystallization by the solute drag of Nb. Pass strains of 0.15 promoted full recrystallization from the second pass onwards in all the studied steels and led to grain refinement of the austenite with a narrower grain size distribution. This was attributed to the increased driving force for recrystallization and increased interfacial area per unit volume which provides nucleation sites for the recrystallized grains at a strain of 0.15 compared to that of 0.07 and 0.1. The extent of grain refinement was found to decrease with decreasing initial grain size and increasing strain. The C-Mn steel, with the coarsest reheat grain size showed the most grain refinement response as compared to the fine grained microalloyed steels. This indicated that a saturation point is reached where no further grain refinement occurs under the studied conditions. Grain refinement of the austenite was due to static recrystallization occurring in the inter-pass times for strains of 0.07 and 0.1. Strains of 0.15 are thresholds where both static and metadynamic recrystallization may be responsible for grain refinement. An optimum uniform austenite microstructure was found when pass strains were at least 0.15. FEM showed that the equivalent plastic strain is concentrated at the centre of an axisymmetrical compressed sample and at the sub-surface of a flat hot-rolled plate [97]. A linear relationship between the applied strain and the equivalent plastic strain at various locations in both the axisymmetrical compressed sample and flat rolled plate was found. Recrystallization models correlated poorly with the measured recrystallized volume fraction in this work at strains of 0.07 to 0.1. This is primarily due to the fact that the recrystallized grain size is exaggerated at small strains due to a division by a smaller number in the models and this therefore leads to an erroneous time for 50 % recrystallization. The other factor can be that at small strains recrystallization may be occurring through the SIBM (Strain induced boundary migration) process and not the classical nucleation theory as illustrated by the models. In the case of microalloyed steels the account of recrystallization retardation by solute Nb in the models may have led to the discrepancy by over-compensating for this retardation.