Effect of rolling temperature on the deformation and recrystallization textures of warm-rolled steels
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I. INTRODUCTION
STEELMAKERS have begun to take an interest in warm (ferritic) rolling, as it has the potential to broaden the product range and decrease the cost of hot-rolled strip materials.[1,2] However, rolling in the ferrite phase (for steels containing carbon or nitrogen in solution) can lead to significant dynamic strain aging (DSA), which, in turn, influences their strain-rate sensitivity (m). In-grain shear bands constitute a form of localized flow within a grain, which occurs at a higher strain rate than that of the bulk material. Therefore, their formation is highly influenced by the strain-rate sensitivity. A negative rate sensitivity will enhance the formation of shear bands and, conversely, an unusually high rate sensitivity can suppress their formation within the grain.[3] The formation of shear bands during warm rolling is generally necessary for the development of strong {111} recrystallization textures, which ensure high mean r values and, hence, good formability.[4] It has been shown that interstitial-free (IF) steels have small positive rate sensitivities across a wide range of temperatures.[5] By contrast, low-carbon (LC) steels can exhibit both negative and high-positive rate sensitivities due to DSA effects. Thus, warm-rolled LC steels tend to have weak {111} annealed textures, which are responsible for the poor formabilities of final products.
Previous workers have shown that alloying additions, such as chromium, can modify the behavior of LC steels by extending DSA effects into the warm-rolling temperature region (650 °C to 800 °C).[6,7] This creates a plateau in the flow stress vs temperature plot rather than the sharp peak usually associated with conventional carbon-nitrogen DSA behavior. A small increase in strain rate does not, therefore, result in a large increase in flow stress within this plateau region, i.e., the rate sensitivity of the material is reduced. It has been found that warm-rolled and annealed LC steels containing 1.3 pct chromium have strong normal direction (ND) fiber-texture components, leading to improved formability.[5] However, for further economic benefit, the possibility of using lower levels of chromium for warmrolled LC steels was studied in the present work. Since the deformation microstructure (particularly, the in-grain shear bands) and the deformation texture influence the orientations of the recrystallized grains,[3,4] the effect of rolling temperature on the development of shear bands and of the deformation and recrystallization textures was studied for various grades of IF and LC steels. Furthermore, the effects of rolling reduction and postrolling aging treatment upon the recrystallization texture were also investigated.
II. EXPERIMENTAL PROCEDURE MOHAMMAD R. TOROGHINEJAD, PhD Student, FAKHRADDIN ASHRAFIZADEH, Professor, and ABBAS NAJAFIZADEH, Professor, are with the Department of Materials Engineering, Isfahan University of Technology, Isfahan 84154, Iran. ALAN O. HUMPHREYS, Research Associate, DONGSHENG LIU, Research Associate, and JOHN J. JONAS, Prof
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