Ultra-Rapid Intercritical Annealing to Improve Deep Drawability of Low-Carbon, Al-Killed Steels
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INTRODUCTION
THE deep drawability of low-carbon, Al-killed steel sheets is related strongly to their recrystallization texture: The development of a strong {111}huvwi texture component during annealing is required to obtain a high value of the mean plastic strain ratio (rm) and, subsequently, a good deep drawability. It has long been established that good deep-drawing properties (rm value ‡ 1.5) can be achieved in low-carbon, Al-killed steels by combining: (1) a low coiling temperature and a batch annealing or (2) a high coiling temperature and a continuous annealing.[1–4] The nucleation of recrystallization is retarded in the first case (low coiling temperature and batch annealing) as a result of the interaction between AlN precipitates (or clusters) and recrystallization during annealing, because of the combined effect of the presence of nitrogen in solution before annealing and the slow heating rate (0.01 K/s).[5] This phenomenon is generally considered as being responsible for a strengthening of the {111}huvwi texture component leading to high rm values on the order of 1.8. In the second case (high coiling temperature and continuous annealing), rm values on the order of 1.5, which is lower than those obtained with a batch annealing, are generally obtained.[2] This occurs because nitrogen is precipitated V. MASSARDIER, D. FABREGUE, and S. CAZOTTES, Assistant Professors, and J. MERLIN, Professor Emeritus, are with the Universite´ de Lyon, CNRS, INSA-Lyon, MATEIS UMR5510, F-69621 Villeurbanne, France. Contact e-mail: Veronique.massardier@ insa-lyon.fr A. NGANSOP, Doctor, is with the INSA-Lyon, MATEIS UMR5510, F-69621 Villeurbanne, France, with the ADEME, BP 90406 49004 Angers Cedex 01, France. Manuscript submitted December 20, 2010. Article published online February 25, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
in the form of AlN nitrides before annealing, and no interaction between AlN precipitation and recrystallization, as with the first case, may occur. To favor the development of the {111}huvwi texture component and the associated better deep drawability, it is necessary to promote the nucleation of grains with this orientation during recrystallization and to make them grow as much as possible. Knowing that the presence of carbon in solution is detrimental for the development of the {111}huvwi texture component, especially in the presence of manganese,[2–4,6,7] the carbon content in solution needs to be reduced as much as possible during recrystallization. This desired carbon condition requires the presence of coarse iron carbides before annealing, which is the case when the steel was coiled at a high temperature. Because the dissolution of the iron carbides is slow during continuous annealing, numerous nuclei with the {111}huvwi orientation tend to develop naturally in the zones where the carbon content in solution is low. It has been shown that intercritical annealing can be used to improve the deep drawability of continuously annealed steels.[2] In this case, recrystallization begins during heating with
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