The production of ultrafine ferrite in low-carbon steel by strain-induced transformation
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I. INTRODUCTION
ONE of the key goals in the development of steels has been to refine the ferrite grain size, as this leads to increases in both strength and toughness. Historically, this has been achieved by normalizing or by cold rolling and annealing to produce fine-grained steels. In recent years, thermomechanical processing has been used to control both the state of the austenite prior to transformation and the transformation history. Currently, the optimum reduction in ferrite grain size is achieved through heavy controlled rolling combined with accelerated cooling. For a simple steel composition in plate rolling, this can reduce the ferrite grain size from 10 m for hot rolling and air cooling to 5 m for controlled rolling and water cooling, resulting in an increase of 80 MPa in the yield strength of the steel. Previous studies related to the austenite-to-ferrite transformation in controlled rolled steels have shown that there is a limiting ferrite grain size of approximately 5 m regardless of the level of retained strain introduced into the austenite.[1] Priestner and Hodgson[2] have discussed how this is at least partly due to ferrite coarsening during transformation. This process involves ferrite grains nucleating on the austenite grain boundaries, followed by the growth of ferrite rafts/ films into the austenite grain interior. This coarsening was particularly pronounced at high levels of retained strain where full ferrite-to-ferrite impingement along an austenite boundary was observed when the ferrite grains were only 1 m, yet the final average ferrite grain size was 5 to 7 m. If the 1-m grain size, here called ultrafine ferrite (UFF), M.R. HICKSON, formerly with BHP Research, Melbourne Laboratories, Mulgrave, 3170, Australia, is Senior Materials Engineer, Metallic Materials, Holden Ltd., Port Melbourne, 3207, Australia. P.J. HURLEY, formerly Postgraduate Student, Department of Materials Engineering, Monash University, Clayton, 3168, Australia, is Postdoctoral Fellow, The Manchester Materials Science Centre, University of Manchester/UMIST, Manchester, United Kingdom. R.K. GIBBS, formerly with BHP Research, Melbourne Laboratories, is Supply Chain Manager, Coated Steel Australia, BHP Steel, Hastings, 3915, Australia. G.L. KELLY, Research Academic, and P.D. HODGSON, Professor of Engineering, are with the School of Engineering and Technology, Deakin University, Geelong, 3217, Australia. Contact email: [email protected] Manuscript submitted February 5, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
could be retained, then a Hall-Petch analysis suggests that this would increase the yield strength by almost 350 MPa compared to a 5-m ferrite microstructure. At the same time there has been some evidence that straininduced transformation (i.e., transformation during rather than after deformation) could lead to much finer ferrite grains than those suggested above. Priestner and co-workers[3,4,5] observed local areas of very fine grains in intercritically rolled steels. The strain-induced ferrite formed
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