Influence of Strain Rate on Interpass Softening During the Simulated Warm Rolling of Interstitial-Free Steels P.R. CETLIN, S. YUE, J.J. JONAS, and T.M. MACCAGNO Most laboratory simulations of hot rolling involve a scaling down of the strain rate from the much higher industrial levels. This leads to slower softening between each rolling pass, for which corrections must be made. In the present work, torsion testing simulations of "warm" rod rolling were conducted on a Ti-Nb interstitial-free (IF) steel at 840 ~ and 770 ~ (i.e., in the ferrite range). For this purpose, "strain rate corrected" interpass times were used, in addition to the more familiar corrections for the stress. The results are compared with those obtained from simulations using uncorrected industrial interpass times. At 840 ~ simulations using corrected interpass times led to high levels of softening between the stages of rolling, thus triggering the reinitiation of cycles of dynamic recrystallization. The initially high stress level at the start of these cycles was responsible for the large differences in the pass-to-pass mean flow stress behavior, compared with that observed when using uncorrected industrial interpass times, or continuous deformations. The differences were much less pronounced at 770 ~ where the rate of softening is much slower than at 840 ~ Predictions for softening based on the Avrami equation underestimated the softening observed using the continuous and uncorrected industrial interpass time schedules and overestimated it for the corrected ones. The former is due to the occurrence of recovery, which is not addressed by the Avrami relation, while the latter is due to the precipitation that takes place during the corrected (longer) interpass times. It was also found that simulations using continuous deformations are applicable only if the interpass softening that would be expected using the corrected interpass times does not exceed about 20 pct. I. INTRODUCTION HOT-WORKING experiments carried out by trials in the mill are difficult and expensive to perform and can pose considerable risks to production equipment. An al- ternative is the laboratory simulation of hot-working pro- cesses, which can provide a great deal of useful information for optimizing microstructure, properties, and processing parameters. This has been carried out exten- sively with the aid of hot torsion machines, t'-tq various compression-type simulators, t12-~7] and reduced-scale rolling mills, t'z,~5,18,191 Such experiments have been em- ployed to simulate s t r i p , t1"5"61 seamless t u b e , 12I and r o d ~3'4"71 rolling and ideally should have faithfully reproduced the industrial conditions. This was largely the case with re- spect to temperature, strain per pass, and interpass time. However, since laboratory equipment is usually unable to apply high industrial strain rates (e.g., up to 1000 s -1 in rod rolling), there was a scaling down of the strain rate in most of these simulations. The lower laboratory strain rates lead to lower stress levels and to coarser microstructures than those prevail- ing under industrial conditions, t5'6~but corrections can be made to allow for these effects. The lower strain rates, however, also retard the softening kinetics of the ma- terial; I2~ thus, the amount of industrial softening pre- dicted from laboratory simulations can be too low. One P.R. CETLIN, on leave from the Departmento de Engenharia Metahirgica, Universidade Federal de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil, and J.J. JONAS, Professors, S. YUE, Associate Professor, and T.M. MACCAGNO, Research Associate, are with the Department of Metallurgical Engineering, McGill University, Montreal, QC, Canada H3A 2A7. Manuscript submitted August 25, 1992. way of overcoming this problem is to increase the lab- oratory interpass times to compensate for the slower rate of laboratory softening. Consider, for example, the strain rate (~) dependence of the time for 50 pct softening (tso) after dynamic recrystallization: t2~ ts0 oc 1/~" [1] For C-Mn steels, n = 0.6; t2m thus, an allowance can be made for a 100-fold decrease in the strain rate, from the industrial level to the laboratory one, by increasing the laboratory interpass times by a factor of 16 over the in- dustrial ones. The objective of the present work was to investigate the suitability of using such a strain rate correction for interpass softening, in addition to the strain rate correc- tion for stress level, in the simulation of rod rolling. This involved, first, an analysis of the kinetics of interpass softening, followed by laboratory hot-rolling simula- tions. Hot torsion testing was selected as the means of simulation because of the ease with which high strains, and multiple-step deformations, can be achieved. I'-'u The experiments were performed in the ferrite range on a Ti- Nb interstitial-free (IF) steel, and all four stages of rod rolling were investigated: roughing, intermediate rolling, prefinishing, and finishing. The approach taken was to compare simulations using uncorrected industrial inter- pass times with those using interpass times corrected for the differences in strain rate between the mill and lab- oratory conditions. Since it has been suggested that multiple-step deformations with very short interpass times (e.g., rod mill simulations) lead to interrupted stress vs strain curves that are similar to continuous ones, c~2J re- sults were also obtained for simulations using continuous deformation. METALLURGICAL TRANSACTIONS A VOLUME 24A, JULY 1993--1543