Comparison of the Annealing Behavior of Three Cold Rolled (Ti-bearing, Nb-bearing and V-bearing) HSLA Steels
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Comparison of the Annealing Behavior of Three Cold Rolled (Ti-bearing, Nb-bearing and V-bearing) HSLA Steels R. Ordonez1, A.J. DeArdo1, 2 and C. Isaac Garcia1 1
Mechanical Engineering and Materials Science Department, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA 2 Basic Metals Processing Research Institute, Mechanical Engineering and Materials Science Department, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, USA ABSTRACT The annealing behavior of three HSLA steels is studied using the combined techniques of EBSDKAM and Sub-grain Method. These techniques have been successfully used to assess the annealing behavior of AK, IF and other high strength steels. Stored energy maps in the hot band, cold rolled and after annealing are constructed and analyzed. The combined usage of the Subgrain Method and EBSD-KAM techniques are employed to calculate and compare the evolution of the stored energy and recrystallization behaviour during the annealing of Ti-bearing, Nbbearing, and V-bearing HSLA steels. Orientation dependent stored energy distribution maps at different annealing stages are constructed and analysed. The results show that the stored energy distribution through the thickness of the samples is not uniform and is independent of the steel composition. Similarly the recrystallization behaviour is strongly related to the initial microstructural condition and particularly to the grain boundary character distribution of the steels. INTRODUCTION The mechanical properties of hot rolled coil products are often not uniform throughout the length of the coil. The variability in properties is very common in steel systems which are dependent on a time-temperature relationship to develop their properties. For example, the hot band microstructure and kinetics of precipitation in a Nb-bearing steel is different from that of a Ti-bearing and/or V-bearing steels under similar processing conditions of finishing rolling temperature, cooling rate and coiling temperatures. The resulting hot band microstructure and the associated stored energy distribution would be expected to be different among the three types of steel systems. In addition, it seems that one of the most important differences between the three systems is the grain boundary character distribution (GBCD). The initial differences in the stored energy become almost negligible when the steels undergo similar amount of cold deformation prior to annealing. However, the initial GBCD does not change after cold rolling. It is wellknown that the major factors responsible for the kinetics of recrystallization during annealing are; 1) stored energy, 2) grain boundary structure, 3) solute drag, 4) pinning force due to precipitation, 5) dislocations and type of defects, 6) crystallographic texture (α and γ fibers), 7) annealing temperature, 8) annealing time, and 9) heating rate.[1-7] The work that is conducted in this study attempts to gain a better understanding of the major factors responsible for the annealing behavior of the three different HSLA steel s
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