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

doi:10.1007/s40195-020-01121-0

Abstract

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 back-scattered 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.

Key words: Grain refinement, Carbide, Pearlitic steel, Cold rolling

Han Zheng, Liming Fu, Xinbo Ji, Ziyong Li, Yanle Sun, Sixin Zhao, Wei Wang, Aidang Shan. Granular Carbides-Assisted Ultrafine-Ferrite Fabrication in the Pearlitic Steel Without Severe Plastic Deformation and Annealing[J]. Acta Metallurgica Sinica (English Letters), 2020, 33(12): 1645-1656.

Figures/Tables 13

Fig. 1 Phase profile with temperature of the experimental steel (carbon content: 0.69 wt%) in equilibrium condition calculated by JMat-Pro V8 software a, corresponding heat treatment and rolling process b

Fig. 2 Different magnification SEM images of pearlitic steel cold rolled to strain of ε=0 a, b, c, ε=0.6 d, e, f, ε=1.4 g, h, i

Fig. 3 TEM images of carbides in P0: a degenerated lamellar cementite and granular M3C with inserted diffraction pattern after heat treatment process, b dark field of M3C, c and d high-resolution transmission electron microscopy (HRTEM) images of M23C6 and its corresponding fast Fourier transform (FFT) patterns on lower right corner, e bright field (BF) of scanning transmission electron microscopy (STEM) micrograph, f high-angle annular dark-field (HAADF) image and EDX elemental mappings of Fe and Cr elements below

Fig. 4 Combined maps of RD-IPF, boundary maps, band contrast and KAM obtained from P0 a, b, CR4 c, d, CR7 steels e-h; S1: subset with more GCs which is framed in black dotted line, S2: subset with few GCs which is framed in red dotted line

Fig. 5 Comparison between P0, CR4 and CR7 steel involving in grain size distribution a, grain boundary distribution b

Fig. 6 Comparison between whole area and local regions (subset 1, subset 2) of CR7 steel involving in grain size distribution a, grain boundary distribution b, KAM c

Table 1 Average values of the microstructural parameters for three different regions in CR7 steel

Regions	LAGBs (%)			HAGBs (%)	Grain size (nm)
Regions	2°-5°	5°-10°	10°-15°	15°-180°	Subgrain	Grain
Subset 1	14.6	18.6	13	53.8	167	388
Subset 2	22.4	27	14.9	35.8	201	810
All	19.4	26.6	18.2	35.8	120	651

Fig. 7 Typical TEM images and corresponding selected area electron diffraction (SAED) patterns of P0 a, b; CR4 c-e

Fig. 8 Typical TEM images, corresponding SAED patterns and STEM images of CR7 steel: a ultrafine structures generated after cold rolling, b bright field (BF) of ultrafine ferrite accompanied by granular carbide and corresponding SAED patterns, c, d dark field of ultrafine ferrite and M23C6, e, f BF and HAADF images of the deformed structure with linear carbide, g, h BF and HAADF images of the deformed structure with elliptic carbide

Fig. 9 Schematic diagram of ultrafine ferrite generated in pearlitic colony during cold rolling

Fig. 10 Schematic diagram of two refinement modes of ferrite in pearlitic colony during cold rolling

Fig. 11 Engineering stress-strain curves of P0, CR4 and CR7 steels

Table 2 Calculated and summarized values of well-assorted mechanical properties from P0, CR4 and CR7 steels

Samples	YS (MPa)	UTS (MPa)	TE (%)
P0	474±4	923±3	13.9±0.9
CR4	775±5	1033±8	2.3±0.2
CR7	1253±11	1465±11	3.3±0.3

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