Ferrite Grain Refinement, Grain Size Distribution, and Texture After Thermomechanical Processing and Continuous Cooling
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THE refinement of ferrite grain size to an ultrafine range significantly improves the strength and toughness of low-carbon steels, which is desired for their application in construction, line pipe, automotive, naval, and defence. Several studies have been carried out on the formation of an ultrafine ferrite grain (ULFG) structure in steel[1–17] indicating that deformation of metastable austenite below Ae3 can significantly refine the ferrite grain size primarily through dynamic SUDIPTA PATRA is with the Indian Institute of Technology Kharagpur, Kharagpur 721 302, West Bengal, India and also with Jindal Stainless Limited, Hisar 125 005, Haryana, India. Contact e-mail: [email protected] ABHISEK MANDAL, RAHUL MITRA, and DEBALAY CHAKRABARTI are with the Indian Institute of Technology Kharagpur. MADHUMANTI MANDAL is with the Indian Institute of Technology Kharagpur and also with the The University of British Columbia, Vancouver, BC V6T 1Z4, Canada. VINOD KUMAR is with the Research and Development Centre for Iron and Steel, RDCIS, Steel Authority of India, Ranchi 834002, Jharkhand, India. Manuscript submitted May 27, 2018. Article published online November 30, 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A
strain-induced austenite to ferrite transformation, DSIT.[1,3–9,14–24] It is possible to deform below Ae3 during the finishing passes of a thermomechanical processing schedule during commercial production.[18–23] In general for low-C ferritic steels, the preferred deformation temperature for creating the ULFG is between Ae3 and Ar3, where Ar3 stands for the actual austenite to ferrite transformation start temperature during cooling.[3–9,14–17] Intercritical deformation between Ar3 and Ar1 can also achieve grain refinement depending on the deformation temperature, applied strain, and strain rate.[1,5,9,24] Grain refinement is maintained during deformation of the transformed ferrite where recovery and continuous dynamic recrystallization (CDRX) retains the ULFG without grain growth.[5–7,25–27] In relatively high alloy and hardenable steels, where austenite remains stable to lower temperatures, deformation at a low temperature 673 K to 873 K (400 C to 600 C) followed by rapid annealing at ~ 1073 K (800 C) was reported to develop ULFG structures by a static strain-induced transformation process, SSIT.[28–30] Irrespective of the transformation mechanism, severe deformation of
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metastable austenite, preferably using a single deformation pass, was found to develop a ULFG structure.[3–9,14–17] Achieving a similar extent of grain refinement using a multi-pass deformation schedule (which is difficult because of static recrystallization and grain growth [19–23]) would be an advantage for industrial processing where large deformation strains in a single pass at relatively low temperatures are not possible. A comparative study is, therefore, necessary to understand the difference in grain refinement between single-pass and multi-pass deformation processes. In general, ferrite grain refinement is quantifie
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