Granular Carbides-Assisted Ultrafine-Ferrite Fabrication in the Pearlitic Steel Without Severe Plastic Deformation and A

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Granular Carbides‑Assisted Ultrafine‑Ferrite Fabrication in the Pearlitic Steel Without Severe Plastic Deformation and Annealing Han Zheng1 · Liming Fu1 · Xinbo Ji1 · Ziyong Li1 · Yanle Sun1 · Sixin Zhao2 · Wei Wang · Aidang Shan1 Received: 22 March 2020 / Revised: 29 April 2020 / Accepted: 21 May 2020 © The Chinese Society for Metals (CSM) and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract The evolution of ferrite grain and cementite lamella during cold rolling in a granular carbide–pearlite steel has been investigated. Particular attention has been given to a quantitative characterization of changes in the ferrite grains. Electron backscattered diffraction and transmission electron microscopy observations show that the ultrafine ferrite (~ 388 nm) can be produced through low equivalent strain cold rolling without severe plastic deformation (SPD) and annealing. The average grain size of ferrite depends on the volume fraction, shape and distribution of granular carbides as well as interlamellar spacing of pearlite. A general explanation of granular carbides-assisted grain refinement is that the embedded carbides between natural barrier will significantly facilitate dislocation nucleation during cold rolling. Dislocation reaction occurs more drastically and quickly near these granular carbides. Such reactions promote the formation of high-angle grain boundaries. The formation of ultrafine ferrite grains and subgrains in steel after cold rolling to ε = 1.4 strain makes the strength and ductility increased simultaneously compared with ε = 0.6 cold-rolled steel. The results suggest a new material design strategy to obtain ultrafine-grained structure via the granular carbides assistance. Keywords  Grain refinement · Carbide · Pearlitic steel · Cold rolling

1 Introduction Grain refinement is usually the focus of attention on the microstructural control of steel [1–10]. In order to fabricate ultrafine-grained structures, it is suggested that an equivalent strain of over 4 generated from severe plastic deformation (SPD) is needed [3]. In other words, the ultrafine grains cannot be fabricated without the support of imposed enormous logarithmic strain [3–5, 11–13]. In general, the efficiency of grain refinement is not only related to the external strain conditions, but also depends on the internal characteristics of material, such as initial microstructures [14, 15]. Tsuji et al. Available online athttp://link.sprin​ger.com/journ​al/40195​. * Liming Fu [email protected] * Aidang Shan [email protected] 1



School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China



Central Research Institute (R&D Center) , Baoshan Iron & Steel Co., LTD, Shanghai, China

2

[16] reported that without large logarithmic strain, an nanostructure (~ 400 nm) austenitic steel with very low stacking fault energy can also be obtained through heavy cold rolling and annealing. Then, the question emerges that whether there exists a way to produce ultrafine ferrite grains in steel plate w