Grain refinement of non-magnetic austenitic steels during asymmetrical hot rolling process

doi:10.1016/j.jmst.2017.06.002

摘要/Abstract

Abstract:

Asymmetrical hot rolling (ASHR) was proposed to acquire productive grain refinement for Fe-20Mn-4Al-0.3C and Fe-18Cr-18Mn-0.5N non-magnetic austenitic steels. The intensive additional shear deformation caused by ASHR promotes the nucleation of recrystallization and grain refining of steel plates. With the speed ratio of 1.2, the austenitic grains were refined to ~5 μm on the surface, the recrystallization fraction was enhanced to ~34.7%, and the thickness of fine-grained surface layer increases to ~450 μm for Fe-20Mn-4Al-0.3C steel. The Fe-18Cr-18Mn-0.5N steel also exhibited an effective surface grain refinement with an average size of ~3 μm, and the recrystallization fraction reached ~76.9% at the speed ratio of 1.15.

Key words: Austenitic steels, Microstructure, Recrystallization, Grain refining, Asymmetrical hot rolling

. [J]. 材料科学与技术, 2017, 33(12): 1572-1576.

Li Changsheng, Ma Biao, Song Yanlei, Zheng Jianjun, Wang Jikai. Grain refinement of non-magnetic austenitic steels during asymmetrical hot rolling process[J]. J. Mater. Sci. Technol., 2017, 33(12): 1572-1576.

图/表 9

Table 1 Chemical compositions of experimental steels.

	C	Si	Mn	P	S	Al	Cr	N	V	Fe
Fe-20Mn-4Al-0.3C	0.298	0.45	20.56	0.006	0.002	4.11	-	0.015	0.11	Bal.
Fe-18Mn-18Cr-0.5N	0.09	0.30	18.32	0.014	0.003	-	18.11	0.48	0.295	Bal.

Table 2 Rolling parameters of experimental steels.

Sample		γ	T_H (°C)	T_S (°C)	T_F (°C)	R_r (%)
Fe-20Mn-4Al-0.3C	N121	1.0	1100	1000	940	60
	N122	1.1	1100	1000	940	60
	N123	1.2	1100	1000	940	60
	N113	1.2	1100	1050	1010	60
	N131	1.2	970	930	830	20
	N132	1.2	970	930	830	40
	N133	1.2	970	930	830	60
	N141	1.0	970	860	750	60
	N142	1.1	970	860	750	60
	N143	1.2	970	860	750	60
Fe-18Mn-18Cr-0.5N	N213	1.15	970	860	750	50

Fig. 1. Microstructures on the upper surface and at the center of N121, N122, N123, N113, N133, N143, N131 and N132 steel during ASHR process (RD: rolling direction, scale bars for a1-d1 and a2-d2 are 100 μm, scale bars for e1-h1 and e2-h2 are 50 μm).

Fig. 2. Schematic illustration of physical metallurgical variation of N123 steel in cross shear region. The completely recrystallized fine grains are generated in the surface layer, and the mixture of deformed austenite and partially recrystallized grains are present together in the central layer of ASHR plate (v1: circular velocity of upper work roll; v2: circular velocity of bottom work roll).

Fig. 3 shows the Vickers microhardness distribution of ASHR plates, corresponding to N121, N122, N123, N141, N142, N143, respectively. An interesting phenomenon can be seen from Fig. 3(a) that the Vickers microhardness distribution along the thickness direction is in the form of parabola shape for N121, N122 and N123. It indicates that the fine grains on the surface of steel plate have been produced by intense shear deformation, and the Vickers microhardness of the surface layer is obviously lower than that of the center. Fig. 3(b) shows the distribution of Vickers microhardness with inverse parabola shape along the thickness direction for N141, N142 and N143. When the deformation temperature is below critical recrystallization temperature [17], the deformation work is stored in the form of vacancies, dislocation or other defects, instead of being released via recrystallization. Hence, the measured results of the Vickers microhardness are well in accordance with the grain size gradient distribution through the thickness of ASHR plates.

Fig. 3. Microhardness distribution along thickness direction of (a) N121, N122, N123 steels at rolling temperature of 1000 °C and (b) N141, N142, N143 steels at rolling temperature of 860 °C.

Fig. 4. Engineering stress-engineering strain curves of conventionally and asymmetrically hot rolled steel N123.

Fig. 5. Shear stress fields for steels (a) N121 and (b) N123 steel.

Fig. 6. Image quality maps with grain boundaries on surface (a) and center (b) of N213 steel.

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