Effects of Modified Inclusions and Precipitates Alloyed by Rare Earth Element on Corrosion and Impact Properties in Low Alloy Steel

doi:10.1007/s40195-022-01404-8

Abstract

Abstract:

This study has focused on the morphology and distribution of inclusions and precipitates modified by rare earth (RE) elements, which has a decisive influence on microstructure, corrosion properties and impact behaviors in Q355 low alloy steel. Characterized by the method of electrolytic extraction and ASPEX-scanning electron microscopy (ASPEX-SEM), small-sized spherical RE inclusions have been modified to replace elongated MnS and large-sized Al₂O₃. Q355RE steel after RE alloying has lower corrosion rate and higher value of α/γ, due to the formation of stable and dense rust layers. Q355RE steel also exhibits better resistance to fracture at low temperature, owing to the presence of RE modification to inclusions and its effects on reducing crack initiation and propagation. Nano-scale RE precipitates containing sulfur and phosphorus is observed along grain boundaries by transmission electron microscopy (TEM). The purification of grain boundaries by RE is beneficial to the improvement of corrosion and impact properties.

Key words: Rare earth element, Inclusion, Precipitate, Q355 low alloy steel, Corrosion resistance, Impact toughness

Xin-Tong Lian, Long Chen, Zeng-Wei Fan, Teng-Shi Liu, De-Xiang Xu, Han Dong. Effects of Modified Inclusions and Precipitates Alloyed by Rare Earth Element on Corrosion and Impact Properties in Low Alloy Steel[J]. Acta Metallurgica Sinica (English Letters), 2022, 35(10): 1719-1730.

Figures/Tables 15

Table 1 Chemical composition of experimental steels (mass%)

Sample	C	Si	Mn	P	S	Cr	Ni	Al	Cu	Ce	La
Q355 steel	0.15	0.21	1.26	0.014	0.007	0.024	0.013	0.049	0.021	-	-
Q355RE steel	0.15	0.22	1.25	0.014	0.007	0.023	0.013	0.049	0.021	0.0144	0.0064

Fig. 1 Schematic illustration of an electrolytic apparatus used for separating inclusions

Fig. 2 Optical micrographs in a Q355 steel, b Q355RE steel

Fig. 3 SEM images and EDS mapping results of inclusions: a oxide, b complex inclusions in Q355 steel; and c RE oxy-sulfide, d RE sulfide in Q355RE steel

Fig. 4 Three-dimensional morphologies of inclusions (painted in different colors) obtained by the method of electrolytic extraction in a Q355 steel, b Q355RE steel

Fig. 5 Corrosion rates of Q355 steel and Q355RE steel obtained in wet-dry cycle immersion test

Fig. 6 Evolution of impact properties with temperature in a longitudinal direction, b transverse direction in Q355 steel and Q355RE steel

Table 2 Type, counts and size distribution of inclusions in Q355 steel and Q355RE steel

Sample	Type of inclusions	1-2 μm	2-3 μm	3-4 μm	4-5 μm	5-6 μm	> 6 μm	Total	Percentage (%)	Average size (μm)	Maximum size (μm)
Q355 steel	CaS-MnS	57	59	17	5	4	3	145	39.4	2.4	7.3
	MnS	28	17	10	4	6	7	72	19.6	4.7	7.8
	CaS-Al₂O₃-MnS	5	71	31	6	4	5	122	33.1	3.8	5.4
	Al₂O₃-MnS	0	2	7	9	3	8	29	7.9	4.5	10.3
Q355RE steel	RES	48	76	31	14	5	1	175	48.3	2.7	7.4
	RE₂O₃	32	61	10	2	2	0	107	29.6	2.3	5.7
	RES-RE₂O₃	17	19	14	1	0	0	51	14.1	2.8	5.4
	RES-RE₂O₃-Al₂O₃	6	10	12	1	0	0	29	8.0	3.0	5.3

Fig. 7 Size distribution and ternary phase diagram of inclusions in a, b Q355 steel, c, d Q355RE steel

Fig. 8 SEM surface morphologies of rust layers in a, c Q355 steel; b, d Q355RE steel; e, f RE oxide particles and its corresponding EDS patterns

Fig. 9 XRD patterns of oxide products of Q355 steel and Q355RE steel

Table 3 Relative content of corrosion products by XRD analysis (mass%)

Sample	α-FeOOH	Fe₃O₄/γ-Fe₂O₃	γ-FeOOH	α/γ
Q355 steel	40.8	55.8	2.1	19.4
Q355RE steel	46.2	54.3	1.5	35.5

Fig. 10 SEM fractographs of investigated fractured steels in Q355 steel at the temperature of a 0 °C, c - 60 °C; in Q355RE steel at the temperature of b 0 °C, d - 60 °C

Fig.11 SEM morphologies of crack initiation and EDS analysis of inclusions in the fractured samples at temperature of - 60 °C in a, c Q355 steel; b, d Q355RE steel

Fig.12 a, b Bright-field TEM images of dislocations and precipitates, c scanning tunneling electron microscopy, high-angle annular dark field (STEM-HAADF) image and mapping of nano-scale precipitate in Q355RE steel

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