Mathematical modeling of the hot strip rolling of microalloyed Nb, multiply-alloyed Cr-Mo, and plain C-Mn steels
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I. INTRODUCTION
THE metallurgical features of plate rolling are now largely understood.[1–10] The long interpass times allow complete static recrystallization (SRX) to take place if the steel is being rolled above the interpass recrystallization stop temperature (Tnr), i.e., in the absence of carbonitride precipitation. Plate rolling below Tnr leads to austenite pancaking, because strain-induced precipitation prevents any further SRX from occurring. By contrast, during the very short interpass times involved in rod rolling, neither SRX nor precipitation can take place,[11–14] and, in contrast to plate rolling, the strain accumulation that takes place leads to dynamic recrystallization (DRX) followed by metadynamic recrystallization (MDRX). Thus, the metallurgical characteristics of plate rolling, on the one hand, and of rod rolling, on the other hand, are now fairly clear. From the point of view of interpass time, strip rolling falls between the two processes discussed previously. During the initial passes, when the interpass times are still relatively long, the metallurgical behavior is similar to plate rolling. As the interpass intervals become shorter, the characteristics approach those of rod rolling. Because the classification of strip rolling as being like plate rolling, on the one hand, or like rod rolling, on the other, depends on chemistry and rolling schedule as well as pass number, a relatively simple and generally accepted analysis of this process has not yet
FULVIO SICILIANO, Jr., Research Associate, and JOHN J. JONAS, Professor, are with the Department of Metallurgical Engineering, McGill University, Montreal, PQ, Canada H3A 2B2. Manuscript submitted March 22, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
been achieved. The goal of the present research was, therefore, to characterize strip rolling in terms of softening mechanism, strain accumulation, grain size, and precipitation. Also, the rolling behavior of microalloyed Nb, plain C-Mn, and some Cr-Mo grades will be considered and compared. Another important point that arose from this study is that the levels of Mn and Si present in the steel appear to influence the precipitation behavior during strip rolling. The occurrence of precipitation changes the rolling load and must, therefore, be accurately predicted. Hodgson[15] has carried out an excellent analysis of the different types of mathematical models and has supplied a list of the advantages associated with the application of a particular model to a given practical situation. According to this author, the main advantages are (1) a reduction in the number of mill trials, (2) an evaluation of hardware modifications, (3) the prediction of variables that cannot be measured, (4) an estimation of the effect of interactions, (5) the potential for performance enhancement, and (6) inexpensive research costs. The applicability of any model must be intensively tested in advance, preferably with industrial data. That is the approach that was employed here. After an improved accuracy in predicting the
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