Effects of Pre-Strain and Two-Step Aging on Microstructure and Mechanical Properties of Fe-30Mn-11Al-1.2C Austenitic Low-Density Steel

doi:10.11900/0412.1961.2020.00509

Acta Metall Sin

2022, Vol. 58

Issue (6): 771-780 DOI: 10.11900/0412.1961.2020.00509

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Effects of Pre-Strain and Two-Step Aging on Microstructure and Mechanical Properties of Fe-30Mn-11Al-1.2C Austenitic Low-Density Steel

REN Ping¹, CHEN Xingpin¹(

), WANG Cunyu², YU Feng², CAO Wenquan²

^1.College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
^2.Special Steel Institute, Central Iron and Steel Research Institute, Beijing 100081, China

Cite this article:

REN Ping, CHEN Xingpin, WANG Cunyu, YU Feng, CAO Wenquan. Effects of Pre-Strain and Two-Step Aging on Microstructure and Mechanical Properties of Fe-30Mn-11Al-1.2C Austenitic Low-Density Steel. Acta Metall Sin, 2022, 58(6): 771-780.

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Abstract

Lightweight Fe-Mn-Al-C steels are promising candidates for automobile structural materials and have gained increased scientific and commercial interest owing to their outstanding mechanical properties and low density. To date, several studies have been conducted to illustrate the mechanism of phase transformation, strengthening, and strain hardening under solution and aging state. Moreover, prestrain before aging as a low-cost and simple method to tailor precipitates and control properties has been widely reported; however, it has been barely investigated in the Fe-Mn-Al-C alloy system. Therefore, in this study, the effects of pre-cold rolling and two-step aging on the microstructure and mechanical properties of Fe-30Mn-11Al-1.2C (mass fraction, %) austenitic low-density steel are investigated using EBSD, TEM, and universal testing machine. Results showed that the yield strength (YS) significantly increased via the two-step aging from 580 MPa (at solution state) to 1120 MPa, but the uniform elongation (UE) sharply decreased to approximately 0. However, after the pre-cold rolling and two-step aging, the YS of the material further improved to 1220 MPa, and the UE significantly increased to 18.2%, which implies an improvement in the comprehensive mechanical properties of the material. According to the microstructure analysis, the increase in YS after the two-step aging was caused by the ordering strengthening effect of κ' carbide. Further, the pre-cold rolling could introduce heterogeneous nucleation sites, inducing intragranular precipitation. The combination of the precipitation strengthening of the precipitates and deformation strengthening induced via the pre-cold rolling further increased the YS of the material. Moreover, these intragranular precipitates could improve the work hardening capability, which is the root cause of the high plasticity of materials. This process provides a novel idea for improving the performance of austenitic low-density steels.

Key words: Fe-Mn-Al-C steel low-density steel pre-cold rolling work hardening two-step aging

Received: 18 December 2020

ZTFLH:

TG142.1

Fund: National Natural Science Foundation of China(51871062);National Natural Science Foundation of China(52171105);Fundamental Research Funds for the Central Universities(2020CDJDPT001)

About author: CHEN Xingpin, professor, Tel: (023)65111547, E-mail: xpchen@cqu.edu.cn

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https://www.ams.org.cn/EN/10.11900/0412.1961.2020.00509 OR https://www.ams.org.cn/EN/Y2022/V58/I6/771

Fig.1 Schematics of thermomechanical treatment (WQ, AC, and CR are the abbreviation of water quenching, air cooling, and cold rolling, respectively)
(a) two-steps aging(b) pre-cold rolling and two-step aging

Fig.2 Room-temperature tensile test results of Fe-30Mn-11Al-1.2C steel (The hollow circles indicated the necking points)
(a) engineering stress-strain curves
(b) true stress-strain curves
(c) strain hardening rate curves

Fig.3 EBSD inverse pole figure map of the rolling direction (a), EBSD phase map (b), dark field TEM image of κ′-carbide (c), and selected area electron diffraction (SAED) pattern of κ′-carbide along [001] _γ zone axis (d) of solution state Fe-30Mn-11Al-1.2C steel sample

Fig.4 Dark field TEM images (a, b) and SAED patterns (c, d) along [001] _γ zone axis of κ′-carbide of A1 (a, c) and A1 + A2 (b, d) Fe-30Mn-11Al-1.2C steel samples

Fig.5 Bright field TEM image of CR Fe-30Mn-11Al-1.2C steel sample

Fig.6 Bright field TEM image of sample CR + A1 (a), dark field TEM image of κ′-carbide (b), and SAED pattern of κ′-carbide along [001] _γ_{zone axis (c)}

Fig.7 TEM analyses of CR + A1 + A2 Fe-30Mn-11Al-1.2C steel sample
(a) bright field TEM image of intergranular precipitates
(b) SAED pattern of β-Mn and DO₃-ordered α phases in Fig.7a along [

1 3 ¯ 1

]

D O 3

and [

01 1 ¯

] _β_-Mn zone axes
(c) dark field TEM image and SAED pattern (inset) along [001] _γ zone axis of κ′-carbide
(d) bright field TEM image of intragranular DO₃-ordered α precipitates showed by arrows
(e) magnifying image of intragranular DO₃-ordered α precipitates
(f) SAED pattern of DO₃-ordered α precipitates along [

1 ¯ 1 ¯ 0

]

D O 3

zone axis

Fig.8 Sketches illustrating the microstructure evolution during the thermomechanical process of Fe-30Mn-11Al-1.2C steel (TB—twin boundary)

Fig.9 Schematics of the atomic arrangement of (110) plane of κ′ carbide (a) and the antiphase boundary (APB) (b)

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