Evolution of microstructure and tensile properties of cold-drawn hyper-eutectoid steel wires during post-deformation annealing

doi:10.1016/j.jmst.2019.08.054

Abstract

Abstract:

Manufacturing temperatures of severely cold-drawn hyper-eutectoid steel wires are sufficiently high to influence the mobility of dislocations and alloy elements, thereby affecting the materials’ mechanical properties. Herein, we describe the evolution of microstructure and tensile strength of the as-drawn 3.45 GPa steel wire during post-deformation annealing for 30 min at 150-450 °C. Annealing at 150 °C raised the strength to 3.77 GPa by age-hardening through activation of dislocations pinning by carbon, while further temperature rising up to 450 °C caused a severe loss of strength. It was proved that annealing at 300 and 450 °C destabilizes the lamellar microstructure, promoting the formation of carbon-deficient (Fe,Mn,Cr)₃C-type cementite particles with preferentially rounded and partially faceted hetero-interfaces. Annealing at 450 °C yielded the accumulation of Mn and Cr at the ferrite/particle interfaces, and their concentrations at the interfaces were dependent on the interface structure; i.e., lower concentrations at rounded interfaces (formed through capillarity-driven coarsening of the spheroidized cementite), and higher concentrations at faceted interfaces (that are initially existing in the as-drawn state). Our proof-of-principle observations, supported by thermodynamic calculations and kinetic assessments, provide a pathway for understanding the changes in microstructural and tensile properties during manufacturing of the hyper-eutectoid steel wires.

Key words: Three-dimensional atom probe (3DAP), High resolution transmission electron microscopy (HRTEM), Nano-grained low-carbon steel, Annealing, Tensile behavior

Majid Jafari, Chan-Woo Bang, Jong-Chan Han, Kyeong-Min Kim, Seon-Hyeong Na, Chan-Gyung Park, Byeong-Joo Lee. Evolution of microstructure and tensile properties of cold-drawn hyper-eutectoid steel wires during post-deformation annealing[J]. J. Mater. Sci. Technol., 2020, 41: 1-11.

Figures/Tables 13

Fig. 1. (a, b) TEM micrographs obtained from as-drawn wire microstructure at different magnifications. (c) High-resolution TEM image from microstructure of as-drawn wire and the corresponding selected area diffraction patterns.

Fig. 2. (a) 3D atom map with an iso-concentration surface of 5.5 at% C taken from the as-drawn wire. (b) Carbon concentration in ferrite taken from ROI #1, and 1D concentration profiles of carbon (teal), Mn (blue) and Cr (magenta) along ROI #2 (indicated by red arrow in a) perpendicular to the ferrite/cementite interface. The teal dots in b represent carbon atoms.

Fig. 3. (a, b) TEM micrographs obtained from 150 °C-annealed wire microstructure at different magnifications.

Fig. 4. (a) 3D atom map with an iso-concentration surface of 5.5 at% C taken from 150 °C annealed wire. (b) Two different projections from the area shown in (a) at higher magnification. (c) Carbon concentration in ferrite taken from ROI #1, and 1D concentration profiles of carbon, Mn and Cr along ROI #2 (indicated by red arrow in a) perpendicular to the ferrite/cementite interface.

Fig. 5. (a, b) TEM images from microstructure of 300 °C-annealed wire at different magnifications. (c) High-resolution TEM image indicating a nanosized globular Fe3-xCx particle with a rounded to partly faceted morphology, and the corresponding selected area diffraction pattern.

Fig. 6. The histograms indicating the Fe3-xCx particle size distribution in (a) 300 °C and (b) 450 °C annealed wires.

Fig. 7. (a) Carbon maps of two different tips with an iso-concentration surface of 5.5 at% C, taken from wires annealed at 300 °C. In the two different projection views of tip #1, most carbon atoms are enriched at nanosized particles and some of them are also decorated at ferrite dislocations. (b) Carbon concentration in ferrite taken from ROI #1, and 1D concentration profiles of carbon, Mn, Cr and Si (red) along ROI #2 indicated by red arrow in (a).

Fig. 8. (a, b) TEM images from microstructure of 450 °C annealed wire at different magnifications. (c) STEM and (d) HAADF images from a spheroidized cementite particle together with the corresponding HRTEM observations of regions #1 and #2.

Fig. 9. (a) 3D atom map with an iso-concentration surface of 5.5 at% C taken from 450 °C-annealed wire. (b) Different projection view of atom map in (a) using an iso-concentration surface of 1.0 at% Cr. (c) Concentration profile of carbon along ROI #2, indicated by red arrow in (a). (d) Concentration profiles of carbon, Mn, Cr and Si along ROI #3, outlined by red arrow in b.

Table 1 Calculated driving force (kJ mol-1) for the formation of (Fe,Mn,Cr)3C and Fe3C at 300 and 450 °C.

Temperature (°C)	(Fe,Mn,Cr)₃C	Fe₃C
300	15.1	12.9
450	12.1	11.3

Table 2 Calculated diffusion coefficients of alloying elements in bcc-structure ferrite and cementite at 300 and 450 °C for 30 min (unit: m2 s-1).

Temperature (°C)	Element	Ferrite	Cementite
300	Carbon	2.18 × 10^-14	8.12 × 10^-21
	Mn	1.45 × 10^-29	9.06 × 10^-32
	Cr	1.34 × 10^-29	1.51 × 10^-31
	Si	3.54 × 10^-29	No data
450	Carbon	8.60 × 10^-13	4.08 × 10^-18
	Mn	5.62 × 10^-24	2.31 × 10^-26
	Cr	5.20 × 10^-24	3.85 × 10^-26
	Si	1.34 × 10^-23	No data

Table 3 Calculated diffusion lengths of Cr and Mn in ferrite and cementite at 300 and 450 °C for 30 min (Unit: m).

Temperature (°C)	Ferrite		Cementite
Temperature (°C)	Cr	Mn	Cr	Mn
300	3.10 × 10^-13	3.22 × 10^-13	3.30 × 10^-14	2.56 × 10^-14
450	1. 94 × 10^-10	2.00 × 10^-10	1.66 × 10^-12	1.29 × 10^-11

Fig. 10. The variations of tensile strength and elongation for as-drawn and annealed wires.

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