Effect of Mn Segregation on Mechanical Properties of 0.3C-11Mn-2.7Al-1.8Si-Fe Medium Mn Steel and Its Mechanism

doi:10.11900/0412.1961.2023.00410

Acta Metall Sin

2025, Vol. 61

Issue (7): 1024-1034 DOI: 10.11900/0412.1961.2023.00410

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Effect of Mn Segregation on Mechanical Properties of 0.3C-11Mn-2.7Al-1.8Si-Fe Medium Mn Steel and Its Mechanism

CAI Xingzhou¹, LIU Shengjie¹, ZHANG Yusen¹, LI Xiaolong¹, ZHANG Yuhe¹, ZHANG Wenbin¹, CHEN Lei¹(

), JIN Miao¹^,²

1 Collenge of Mechanical Engineering, Yanshan University, Qinhuangdao 066004, China
2 Tongyu Heavy Industry Co. Ltd., Yucheng 251200, China

Cite this article:

CAI Xingzhou, LIU Shengjie, ZHANG Yusen, LI Xiaolong, ZHANG Yuhe, ZHANG Wenbin, CHEN Lei, JIN Miao. Effect of Mn Segregation on Mechanical Properties of 0.3C-11Mn-2.7Al-1.8Si-Fe Medium Mn Steel and Its Mechanism. Acta Metall Sin, 2025, 61(7): 1024-1034.

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Abstract

Improvements in passenger safety and fuel efficiency are crucial issues in the automotive industry. The use of advanced high-strength steel (AHSS) in automotive parts has been suggested as a solution to these issues because it enables large weight reduction and good crash worthiness. Strength and ductility are the key mechanical properties of automotive AHSS. However, high strength is often accompanied by low ductility, resulting in the so-called strength-ductility trade-off dilemma. Currently, there is an increasing demand for automotive AHSS that exhibits a balance between strength and ductility. Lightweight and high-strength medium Mn steel (MMnS with a Mn mass fraction of 3%-12%), as a representative example of the third-generation automotive AHSS, has an excellent combination of strength and plasticity due to the effective usage of the coupled transformation-induced plasticity (TRIP) effect and twinning-induced plasticity (TWIP) effect of the metastable austenite constituent upon deformation. To further improve comprehensive mechanical properties, MMnS with a high Mn content were developed to increase the austenite fraction. Thus, a duplex structure with an ultrafine ferrite and austenite matrix was formed. However, Mn segregation is likely to occur in MMnS with increasing Mn content, especially in the cases of Mn > 10% (mass fraction), which considerably influences MMnS's performance. Therefore, the effects of Mn segregation on the overall mechanical properties, microstructure, and deformation mechanism of MMnS need to be elucidated in more details. In this paper, the influence of Mn segregation on the microstructure and mechanical properties of MMnS with an austenite-ferrite duplex structure and a nominal composition of 0.3C-11Mn-2.7Al-1.8Si-Fe was systemically investigated. Specifically, the underlying mechanism of Mn segregation that affects the mechanical and microstructural behavior of cold-rolled and annealed MMnS was analyzed. The results show that Mn segregation causes the formation of a Mn-rich banded structure, where the grain size of austenite is larger, austenite stability is higher, and fine ferrite is distributed more sparsely on the austenite matrix compared with the case without Mn segregation. The dominant plastic deformation mechanism of austenite in the non-Mn segregation zone involves martensitic transformation and twinning, leading to the coupled TRIP + TWIP effect, while the rate of martensitic transformation is higher than those without Mn segregation. However, the martensitic transformation is inhibited in the austenite of the Mn-rich structure because of its higher stability, limiting the TRIP effect. Consequently, the test MMnS with Mn segregation shows lower ductility and fracture resistance than those without Mn segregation; moreover, its finer austenite enhances the work-hardening capacity.

Key words: medium Mn steel coarse-grained austenite TRIP + TWIP effect work hardening Mn segregation

Received: 08 October 2023

ZTFLH:

TG142.1

Fund: National Natural Science Foundation of China(52275388);National Natural Science Foundation of China(52075474);Central Guiding Local Science and Technology Development Fund Projects(236Z1008G);Natural Science Foundation of Hebei Province(E2022203206);Cultivation Project for Basic Research and Innovation of Yanshan University(2021LGZD009);Cultivation Project for Basic Research and Innovation of Yanshan University(2022BZZD002)

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	Articles by authors
	CAI Xingzhou
	LIU Shengjie
	ZHANG Yusen
	LI Xiaolong
	ZHANG Yuhe
	ZHANG Wenbin
	CHEN Lei
	JIN Miao

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https://www.ams.org.cn/EN/10.11900/0412.1961.2023.00410 OR https://www.ams.org.cn/EN/Y2025/V61/I7/1024

Fig.1 Calculated phase diagram (a) and schematic of thermomechanical process (b) of 0.3C-11Mn-2.7A1-1.8Si-Fe medium Mn steel

Fig.2 SEM images (a, c) and Mn element distributions (b, d) in the center (a, b) and surface (c, d) of medium Mn steel forged slab (Insets in Figs.2a and c are the schematics of selected positions of the forging block. w_Mn—mass fraction of Mn)

Fig.3 Work-hardening rate curves and true stress-strain curves of the center (S1 represents sample with segregation band) and surface (S2 represents sample without segregation band) samples (ε_s—critical strain of martensitic transformation)

Table 1 Mechanical properties of S1 and S2 samples

Fig.4 SEM images of S1 sample (a) and corresponding EDS line scanning result of Mn (b)

Fig.5 EBSD images (a, c) and inverse pole figures (IPFs) (b, d) of S1 (a, b) and S2 (c, d) samples

Fig.6 Grain size distributions of ferrite (a) and austenite (b) (Inset in Fig.6b is an enlarged graph of austenite grain size in the range of 6-13 μm) in the initial state of S1 and S2 samples

Fig.7 TEM images and corresponding selected area electron diffraction (SAED) patterns (Insets) of deformed substructures of S1 (a-f) and S2 (g-i) samples with different strains (ε)
(a) as-annealed microstructure in the coarse-grained region of S1 sample
(b) dislocation in the coarse-grained region of S1 sample with ε = 20% (c1, c2) bright (c1) and dark (c2) field images for α' martensite in the coarse-grained region of S1 sample with ε = 40% (d) as-annealed microstructure in the fine-grained region of S1 sample (e) α′ martensite nucleated at the intersection of twins in the fine-grained region of S1 sample with ε = 20% (f1, f2) bright (f1) and dark (f2) field images for α' martensite in the fine-grained region of S1 sample with ε = 40% (g) as-annealed microstructure in the S2 sample (h) α′ martensite nucleated at the intersection of twins in the austenite of S2 sample with ε = 20% (i1, i2) bright (i1) and dark (i2) field images for α' martensite in austenite of S1 sample with ε = 60%

Fig.8 EBSD images of the S1 (a, b) and S2 (c, d) samples with strains of 15% (a, c) and 38% (b, d) (RD—rolling direction, TD—tension direction)

Fig.9 Low (a, c) and high (b, d) magnified SEM images of S1 (a, b) and S2 (c, d) samples

Sample	Composition of austenite				Grain size	V_γ	$M d 30$	γ_SFE	Deformation
	(mass fraction / %)				μm	%	^oC	mJ·m^-2	mechanism
	C	Mn	Al	Si	μm	%	^oC	mJ·m^-2	mechanism

S1 (rich Mn)	0.74	13.12	2.12	2.10	8.52	92.4	108.33	10.44	TRIP
S1 (low Mn)	0.73	11.15	2.25	1.90	1.58	73.1	123.84	16.45	TRIP + TWIP
S2	0.73	11.72	2.23	1.93	1.56	69.3	118.91	15.62	TRIP + TWIP

Table 2 Alloying element contents and grain size of austenite and its characteristics in S1 (coarse and fine grain regions) and S2 samples

Fig.10 EBSD images (a, b) and SEM images (c, d) of cracking features near the fracture surface of S1 (a, c) and S2 (b, d) samples

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