Modeling of thermomechanical and metallurgical phenomena in steel strip during hot direct rolling and runout table cooling of thin-cast slabs
Creators: name: Muojekwu, Cornelius Anaedu
Publisher: name: University of British Columbia
Year: 2009

description: The present research was directed at adequate prediction of the temperature, deformation behavior (roll force, flow stress, strain and strain rate) and micro structural evolution (recovery, recrystallization, grain growth, austenite and ferrite grain sizes) during rolling in the Compact Strip Production (CSP) process, as well as the final mechanical properties of the hot rolled strips. This was accomplished with the aid of integrated process modeling, involving mathematical simulation, laboratory experiments and industrial campaigns. The study covered two conventional plain carbon steel grades, the A36 (AISI 1018, 0.17C-0.74Mn) and DQSK (AISI 1005, 0.038C-0.3Mn), and a range of plain carbon steel grades (0.06-0.09 C, 0.16-0.9 Mn) produced at HYLSA's CSP mill at Monterrey, Mexico. In the laboratory, compression tests (both single and double-hits) were carried out on the Gleeble 1500 thermomechanical simulator in order to elucidate the effect of coarse austenite grain size on the flow stress and recrystallization behavior of the plain carbon steels. It was found that coarse grain size not only decreased the flow stress at a given strain but also substantially reduced the tendency toward dynamic recrystallization. An increase in grain size from 244 to 1110 um which is typical of the first stands of a conventional finishing mill and CSP hot-strip mill respectively, resulted in up to a 30 MPa decrease in the flow stress of both A36 and DQSK steel grades at similar operating conditions of temperature, strain and strain rate. In combination with flow stress curves for finer grain sizes, it was found that a distinctive boundary exists between flow curves with peaks and those without peaks, a very important finding that allowed for a novel quantitative delineation of the occurrence of peaks in flow curves for any given set of deformation conditions. It was found that for the range of grain sizes and strain measured, complete recrystallization (Fx&gt; 0.95) occurred between 2 and 4 seconds at 1100 °C and a strain rate of about 5 s⁻¹. The recrystallization kinetics obtained for fine austenite grains were found to be inadequate when extrapolated to the coarse grain size of A36 steel. In order to validate the model and laboratory results with mill measurements from an operating CSP plant, an industrial trial was carried out at HYLSA's CSP mill in Monterrey, Mexico. During the industrial campaign, intermediate temperature measurements were made, CSP slab and coil samples were acquired, and all measured and recorded mill data and practices were obtained. The prior as-cast austenite grain size from one of the slabs was estimated to be 990 um. Analysis of a cobbled strip revealed that it takes the first two stands to break down the as-cast structure and subsequent stands to refine the resulting equiaxed micro structure through recrystallization as in the conventional rolling. This finding is believed to have ramifications for CSP rolling of crack-sensitive grades as well as the emerging technology of strip casting. The ferrite grain size and mechanical properties of CSP strips were found to be dependent on the coiling temperature, strip thickness and steel composition. The final ferrite grain size decreased as the coiling temperature and strip thickness were reduced. Low coiling temperature sometimes lead to a non-polygonal structure as was the case in a 3.15 mm thick strip coiled at 560 °C. The yield and ultimate tensile strengths (YS and UTS) decreased with increasing coiling temperature and strip thickness as well as with reduced carbon equivalent and residual content, while the percent elongation increased. A 25 percent decrease in yield strength, a 10 percent decrease in ultimate tensile strength and a 2 percent increase in percent elongation were found to be associated with the annealing effect of slow cooling of coil bundles for strips coiled at 715 ± 15 °C and left to cool in air. Comprehensive mathematical modeling of the rolling process was carried out employing finite difference and finite element analysis. The CSP mill measurements were utilized to validate model predictions of temperature, roll force, grain size and mechanical properties. Good agreement was obtained between prediction and measurement in most of the cases. An estimate of the heat extraction from the various mill sub-units was conducted from the validated calculations. It was found that heat loss by radiation accounted for 48-51 percent of the total heat loss, the work rolls accounted for 41-44 percent, the descaling unit accounted for 4-6 percent and the interstand sprays accounted for the remaining 3-4 percent. It was found that the uniform strain model consistently predicts lower temperatures than the target exit temperature for thin gauges due to a low estimate of deformation heat. Model results captured the details of heat transfer, deformation, recrystallization and austenite decomposition in the CSP mill. The effect of various mill parameters were elucidated, and the similarities and differences between conventional cold-charge rolling and CSP rolling were highlighted.