Creep Deformation and Dynamic Grain Growth in an Interstitial-Free Steel
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RODUCTION
INTERSTITIAL-FREE (IF) steels, also known as ultra-low-carbon and extra-low-carbon steels, are used in industry because of their high ductilities and formabilities.[1] Formability, especially deep drawability of sheet, can be improved through microstructure control. Small alloying additions of Ti and/or Nb are used to control grain size.[1] Crystallographic texture is controlled through rolling and annealing schedules. A high Lankford coefficient, r, is often used as an indication of deep drawability.[2] A strong c-fiber texture, wherein the h111i is perpendicular to the sheet normal, increases r and improves deep drawability.[3,4] This desirable strong c-fiber texture is developed during recrystallization following heavy cold rolling reductions of steel sheet.[3] Ukena reported that the average Lankford coefficient, r value, was increased beyond that produced by static annealing at 800 C alone when a tensile stress of 20 MPa was applied to a cold-rolled, low-carbon (0.02 wt pct) RYANN E. RUPP is with the Idaho National Laboratory, 1955 N. Fremont Ave., Idaho Falls, ID 83415. PHILIP J. NOELL is with the Sandia National Laboratories, P.O. Box 5800, Albuquerque, NM 87185-0889. ERIC M. TALEFF is with The University of Texas at Austin, Department of Mechanical Engineering, 204 East Dean, Keeton St., Stop C2200, Austin, TX 78712. Contact e-mail: taleff@utexas.edu Manuscript submitted May 7, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Al-killed steel sheet.[5,6] The increase in r from annealing under a tensile stress resulted from an increase in the intensity of the f111gh110i texture component, part of the c-fiber. The cause of this unusual texture change during annealing under tensile stress could not be precisely established. The same value of r 1:9 was produced by annealing times up to 10 s for specimens with and without an applied tensile stress. Because this time is probably sufficient for recrystallization, recrystallization behaviors are unlikely to be the cause of the observed texture difference. The r value increased beyond 1.9 with additional annealing time, and the increase in specimens under tensile stress was more rapid than in unstressed specimens. For 300 s annealing at 800 C, an unstressed specimen achieved r ¼ 2:02 while a stressed specimen achieved r ¼ 2:16. This result points toward two potential causes of the observed texture change: 1. crystal rotation from slip deformation and 2. an increased rate of grain growth during plastic deformation, i.e., dynamic grain growth. The present investigation was undertaken to identify the mechanism that produces this crystallographic texture change. An IF steel with a small Ti addition to provide grain size control was chosen for study. Grain growth is important to this study and is among the several phenomena fundamentally important to the processing of metals and alloys. The grain structures and crystallographic textures developed during grain growth can strongly influence the mechanical and
physical properties of a material. Grain growth in a
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