Microstructural evolution during thermomechanical processing of a Ti-Nb interstitial-free steel just below the Ar 3 temp
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I.
INTRODUCTION
INTERSTITIAL-FREE (IF) steels are state-of-the-art sheet steels, which have exceptionally good deep drawing properties.[1,2] These steels typically have a total C 1 N content of less than 50 ppmw and contain microalloy additions of Nb and/or Ti. Processing is normally carried out in two stages: (1) production of ‘‘hot band’’ by hot rolling, accelerated cooling, and coiling; and (2) cold rolling, annealing, and coating. Warm rolling in the ferrite range is an alternative processing route for producing IF steels having strength and formability intermediate to hot-rolled sheet and cold-rolled/annealed sheet.[3,4] Most of the previous studies of microstructural evolution during thermomechanical processing (TMP) of IF steels pertain to the production of hot band for subsequent cold rolling and annealing. For example, Najafi-Zadeh et al.[5] studied the hot deformation of IF steels, and austenite transformation temperatures have been reported for IF steels,[5] ultra low carbon steels,[6] and dilute Fe alloys.[7] For the range of cooling rates employed in accelerated cooling of sheet, the transformation products are either polygonal ferrite (PF) or massive ferrite (MF).[7] Studies of precipitation in normally processed hot band report large (100 to 1000 nm) TiN, TiS, and (Ti,Nb)4C2S2 and small (10 to 100 nm) TiC and (Ti,Nb)C precipitates.[8,9,10] The MC precipitates often form as coprecipitates on pre-existing nitride, sulfide, or carbosulfide particles. The few studies of ferrite recrystallization in the warm rolling range[3,4] show that the precipitate distribution prior to deformation is an important I.A. RAUF, formerly with the Department of Materials and Metallurgical Engineering, Queen’s University, is Research Associate, Department of Physics, University of Alberta, Edmonton, AB, Canada T6G 2J1. J.D. BOYD, Professor, is with the Department of Materials and Metallurgical Engineering, Queen’s University, Kingston, ON, Canada K7L 3N6. Manuscript submitted August 24, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
factor, and the temperature range for complete recrystallization is very limited. In the present study, austenite transformation characteristics, precipitation behavior, and recrystallization of deformed ferrite were investigated for an IF steel in the temperature range just below Ar3. The objective was to determine the details of microstructural evolution under processing conditions relevant to warm rolling. II.
EXPERIMENTAL
A. Thermomechanical Treatments The material studied was a commercial Ti-Nb IF steel, having the composition given in Table I. The starting material was 25-mm-thick transfer bar, which had been hot rolled and air cooled. Cylindrical dilatometer samples 4mm diameter and 8-mm long were machined directly from the transfer bar with the cylinder axis parallel to the transverse direction of the rolled bar. Laboratory thermomechanical treatments were carried out in a MMC quench-deformation dilatometer. Sample temperature was controlled to 55 7C by a Type S thermocouple spot
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