Hot Deformation Behavior and Processing Maps of a Medium Manganese TRIP Steel

doi:10.1007/s40195-018-0854-x

Abstract

Abstract:

The hot deformation behavior of a medium-Mn steel was studied in terms of hot compression flow curves in the temperature range of 850-1050 °C and strain rates of 0.05-10 s^-1. The thermo-mechanical analysis was carried out and suggested that the microstructure during deformation was completely austenite which had high tendency for dynamic recrystallization (DRX). The flow behavior was characterized by significant flow softening at deformation temperatures of 950-1050 °C and lower strain rates of 0.05-5 s^-1, which was attributed to heating during deformation, DRX and flow instability. A step-by-step calculating procedure for constitutive equations is proposed. The verification of the modified equations indicated that the developed constitutive models could accurately describe the flow softening behavior of studied steel. Additionally, according to the processing maps and microstructure analysis, it suggested that hot working of medium-Mn steel should be carried out at 1050 °C, and the strain rate of 0.05-10 s^-1 resulted in significantly recrystallized microstructures in the in steel. The flow localization is mainly flow instability mechanism for experimental steel.

Key words: Medium-Mn steel, Hot deformation behavior, Constitutive equations, Flow instability mechanism, Recrystallized microstructure

Ning Yan, Hong-Shuang Di, Hui-Qiang Huang, R D. K. Misra., Yong-Gang Deng. Hot Deformation Behavior and Processing Maps of a Medium Manganese TRIP Steel[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(8): 1021-1031.

Figures/Tables 15

Table 1 Chemical compositions of experimental steel (wt%)

Mn	Si	C	P	S	Fe
8.94	2.99	0.22	0.004	0.004	Bal.

Fig. 1 Time-temperature detail of hot compression tests for experimental steel (Ae3: equilibrium transformation temperature from austenite to ferrite)

Fig. 2 Initial microstructure of steel before compression

Fig. 3 Equilibrium phase diagrams a, equilibrium phase constituents b predicted by Thermo-Calc for tested steel

Fig. 4 Flow stress-strain curves of tested steel at different strain rates and temperatures: a 850 °C; b 900 °C; c 950 °C; d 1000 °C; e 1050 °C

Fig. 5 Relationship between \(\ln \sigma - \ln \dot{\varepsilon }\)a, \(\sigma - \ln \dot{\varepsilon }\)b at ε?=?0.2

Fig. 6 Relationship between ln[sinh(ασ)]?-?ln \(\dot{\varepsilon }\)a, ln[sinh(ασ)]?-?10,000/Tb

Fig. 7 Relationship among αa, nb, Qc, ln Ad with true strain

Table 2 Coefficients of polynomial functions

	i?=?0	i?=?1	i?=?2	i?=?3	i?=?4	i?=?5	i?=?6
a _i	0.00472	0.02897	-?0.41598	2.18992	-?5.53854	6.81792	-?3.27803
b _i	17.28371	-?174.879	1354.541	-?5528.39	12,060.81	-?13,328.9	5865.289
c _i	34.38965	-?142.208	2075.295	-?11,380.1	29,302.64	-?35,982.9	17,070.85
d _i	38,1403.3	-?1.68E+6	2.29E+7	-?1.23E+8	3.15E+8	-?3.86E+8	1.83E+8

Fig. 8 Comparison of experimental and calculated data at deformation temperatures of 850 °C a, 900 °C b, 950 °C c, 1000 °C d, 1050 °C e

Fig. 9 Correlation between experimental and calculated data from established equations

Fig. 10 Average absolute relative error (AARE) at different deformation conditions

Fig. 11 Processing maps for alloyed steel at true strains of 0.2 a, 0.3 b, 0.4 c, 0.5 d, 0.6 e

Fig. 12 SEM images of compressed samples deformed at 1050 °C with strain rates of 0.05 s-1a, 1 s-1b, 5 s-1c, 10 s-1d

Fig. 13 SEM images of specimens deformed in instability region: a 900 °C, 10 s-1; b 1000 °C, 5 s-1

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