Modeling recovery and recrystallization kinetics in cold-rolled Ti-Nb stabilized interstitial-free steel
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TRODUCTION
STABILIZED interstitial-free (IF) steel, produced by adding sufficient Ti and/or Nb to an extra-low-carbon grade (;0.005 wt pct C) to precipitate interstitial C and N atoms, is increasingly being used to meet the demand for continuously annealed steel sheet having improved deep drawability and nonaging properties.[1,2] The deep drawability of these steels, which is dependent on both chemistry and thermomechanical processing,[3–6] is due to the preferred crystallographic texture of the sheet that results after cold deformation and annealing.[7,8] Onset and completion of recrystallization have been reported to occur at considerably longer times in cold-rolled IF steels than in other low carbon steels,[6,9] giving more time for recovery processes. However, very little information is available concerning the kinetics of the recovery and recrystallization processes occurring during isothermal and continuous heating annealing treatments. Fully stabilized IF steel is ideally suited for continuous annealing, due to the absence of interstitial C and N and the associated freedom from aging effects.[1,10] Mathematical modeling of the alternative batch or continuous annealing processes requires the quantitative characterization of microstructure evolution during heating at rates as small as K. MUKUNTHAN, Research Engineer, is with BHP Research, New Castle Laboratories, Wallsend, NSW, Australia 2287. E.B. HAWBOLT, Professor, is with the Department of Metals and Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1Z4. Manuscript submitted March 13, 1995. 3410—VOLUME 27A, NOVEMBER 1996
0.025 7C s21 simulating batch annealing to 20 7C s21 for continuous annealing. Although some information regarding the recrystallization kinetics of IF steels can be found in the literature,[3,6,11] there is no reported study in which the recrystallization kinetics were modeled and experimentally verified for this wide range of heating rates. X-ray line broadening, a parameter related primarily to the degree of nonuniform lattice strain,[12] has been used to quantify both recovery and recrystallization effects in lowcarbon steels.[3,13,14] The isothermal recovery kinetics have been described using a logarithmic relationship,[15] whereas the isothermal recrystallization kinetics have been characterized in terms of the Johnson and Mehl,[16] Avrami,[17] Kolmogorov[18] (JMAK) theory and the Speich and Fisher (SF)[19] relationship. An alternative approach involving the microstructural path concept[20,21,22] has also been applied to model the microstructure and the kinetic aspects of recrystallization. The principle of additivity[23,24] has been employed for modeling the nonisothermal kinetics of recrystallization.
II.
EXPERIMENTAL PROCEDURES
The material used in this study was a Ti-rich, Nb-lean IF steel; its chemical composition, as provided by Stelco (Hamilton, ON, Canada) is shown in Table I. The material received was hot band and hot rolled to a finishing temperature of 890 7C (Ar3) and coiled at 600 7C. Th
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