Uncovering Microstructure-Property Relationship in Ni-Alloyed Fe-Mn-Al-C Low-Density Steel Treated by Hot-Rolling and Air-Cooling Process

doi:10.1007/s40195-024-01666-4

Abstract

Abstract:

This paper focuses on the relationship between the microstructure and tensile properties of Fe-Mn-Al-C low-density high-strength steel processes by hot-rolling and air-cooling process. The microstructure analysis reveals that the combination of hot-rolling and air-cooling results in the formation of heterogeneous structures comprising different-sized γ and B2 phases in the low-density steel with the addition of nickel (Ni). The addition of Ni promotes the formation of the B2 phase and induces the pinning of B2 phase particles at the γ grain boundaries. This pinning effect effectively hinders the growth of the γ grains, leading to grain refinement. The tensile test results demonstrate that LDS-5Ni (low-density steel, LDS) exhibits excellent high strength and ductility combination, e.g., a tensile strength of 1535 MPa, yield strength of 1482 MPa, and elongation of 23.3%. These remarkable mechanical properties are primarily attributed to the combined strengthening contributions of grain refinement and duplex nano-sized second-phase precipitation hardening.

Key words: Low-density steel, Microstructure, B2 phase, κ-Carbide, Tensile properties

Xiaoliang Jia, Guhui Gao, Xiaolu Gui, Chun Feng, R. D. K. Misra, Bingzhe Bai. Uncovering Microstructure-Property Relationship in Ni-Alloyed Fe-Mn-Al-C Low-Density Steel Treated by Hot-Rolling and Air-Cooling Process[J]. Acta Metallurgica Sinica (English Letters), 2024, 37(4): 713-725.

Figures/Tables 13

Table 1 Chemical composition (in wt%) of LDS steels used in this study

Specimen	Mn	Al	Ni	C	Fe
LDS-0Ni	21	10.1	-	1.03	Bal.
LDS-5Ni	21.53	10.08	4.9	0.98	Bal.

Fig. 1 Room temperature tensile flow curves and strain hardening behaviors of the LDS-0 Ni and LDS-5 Ni obtained at an initial strain rate of 10-3 s−1. a Engineering stress-strain curves; b true stress-strain curves and strain hardening rate as a function of true strain; c instantaneous work-hardening exponent as a function of true strain

Table 2 Tensile properties of the steel in this study

Specimen	YS (MPa)	UTS (MPa)	Total El. (%)
LDS-0 Ni	1127 ± 3	1149 ± 6	25.0 ± 1.8
LDS-5 Ni	1482 ± 3	1535 ± 5	23.3 ± 1.6

Fig. 2 Microstructures of ND surfaces of LDS-0Ni a, b and LDS-5Ni c, d steels, e XRD patterns

Fig. 3 EBSD phase maps a, c and IPF maps b, d in a, b LDS-0Ni, c, d LDS-5Ni steels

Table 3 Volume fraction (%) and size (μm) of austenite and B2 within the microstructure of the steel

Specimen	Austenite	Total (austenitic and precipitates within the matrix)	B2
Specimen	Austenite	Total (austenitic and precipitates within the matrix)	Particles	Band	Total
LDS-0 Ni	Fraction (%)	100	-
LDS-0 Ni	Average size (μm)	46.9 ± 6.5	-
LDS-5 Ni	Fraction (%)	83.3	10	6.7	16.7
LDS-5 Ni	Average size (μm)	7.0 ± 4.3	3.37 ± 1.3	20.9 ± 7.3	-

Fig. 4 EPMA of LDS-5Ni steel

Table 4 Compositions of austenite and B2 in specimens treated by hot-rolling and air-cooling

Specimen	Phase	Mn	Al	Ni	C
LDS-0Ni	γ	21.97 ± 0.27	9.61 ± 0.19	-	1.09
LDS-5Ni	Γ	22.94 ± 0.16	8.06 ± 0.05	4.14 ± 0.03	1.18
LDS-5Ni	B2	18.32 ± 2.69	11.89 ± 1.99	10.60 ± 3.16	-

Fig. 5 TEM bright-field images and corresponding SAED patterns of LDS-5Ni and LDS-0Ni steels. a LDS-0Ni; b, c LDS-5Ni; d zone axis of [1 1 0]γ//[1 1 0]κ in LDS-0Ni; e zone axis of [1 1 0]γ//[1 1 0]κ in LDS-5Ni; f zone axis of [3 1 1] of B2 in LDS-5Ni; g zone axis of [0 0 1] of ordered B2 in LDS-5Ni

Fig. 6 OM a and EBSD b phase maps of as-homogenized LDS-5Ni steel; c phases fractions calculated by Thermo-Calc of Fe-21Mn-10Al-5Ni-C low-density steel (using TCFE 11 database)

Fig. 7 HRTEM images: a LDS-0Ni and b LDS-5Ni sample, a1, b1 zoom-in images of a, b, respectively, and corresponding FFT patterns, a2, b2 IFFT images for a1 and b1, respectively, c EDS maps of the B2 phase in the LDS-5Ni sample

Table 5 Estimated contributions of austenite and B2 to overall yield strength of steel (MPa)

Specimen	Austenite				B2			\(\sigma_{{{\text{re}}}}\)	\(\sigma_{{{\text{YS}}}}\) (experimental)
Specimen	\(\sigma_{0} + \sigma_{{{\text{ss}}}}\)	\(\sigma_{{{\text{GB}}}}\)	\(\sigma_{{{\text{dis}}}}\)	\(f_{\gamma } \sigma_{\gamma }\)	\(\sigma_{{{\text{B}}2}} {\text{ - particles}}\)	\({\sigma }_{{{\text{B2}}}} {\text{ - band}}\)	\(f_{{{\text{B}}2}} \sigma_{{{\text{B}}2}}\)	\(\sigma_{{{\text{re}}}}\)	\(\sigma_{{{\text{YS}}}}\) (experimental)
LDS-0 Ni	612	68	250	930	-		-	197	1127
LDS-5 Ni	570	175	273	849	866	578	126	507	1482

Fig. 8 Fractographs of LDS-0Ni a, b, LDS-5Ni c, d specimens failed in tension

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