Grain Size-Dependent Mechanical Properties of a High-Manganese Austenitic Steel

doi:10.1007/s40195-018-0828-z

Abstract

Abstract:

The effect of grain size on the mechanical properties of a high-manganese (Mn) austenitic steel was investigated via electron-backscattered diffraction, transmission electron microscope, X-ray diffraction, and tensile and impact tests at 25 °C and - 196 °C. The Hall-Petch strengthening coefficients for the yield strength of the high-Mn austenitic steels were 7.08 MPa mm^0.5 at 25 °C, which increased to 14 MPa mm^0.5 at - 196 °C. The effect that the grain boundary strengthening had on improving the yield strength at - 196 °C was better than that at 25 °C. The impact absorbed energies and the tensile elongations were enhanced with the increased grain size at 25 °C, while they remained nearly unchanged at - 196 °C. The unchanged impact absorbed energies and the tensile elongations were primarily attributed to the emergence of the micro-twin at - 196 °C, which promoted the cleavage fracture in the steels with large-sized grains. Refining the grain size could improve the strength of the high-Mn austenitic steels without impairing their ductility and toughness at low temperature.

Key words: High-Mn, steel, Grain, size, Low-temperature, toughness, Hall-Petch, Micro-twin

Wang Xiao-Jiang, Sun Xin-Jun, Song Cheng, Chen Huan, Tong Shuai, Han Wei, Pan Feng. Grain Size-Dependent Mechanical Properties of a High-Manganese Austenitic Steel[J]. Acta Metallurgica Sinica (English Letters), 2019, 32(6): 746-754.

Figures/Tables 12

Table 1 Chemical compositions of experimental steel (wt%)

C	Si	Mn	S	P	Mo	Al
0.46	0.15	22.28	0.0019	0.0051	0.34	0.028

Fig. 1 EBSD images of austenite grains of experimental steels with various solution treatment processes: a 900 °C, 40 min; b 950 °C, 40 min; c 1000 °C, 40 min; d 1100 °C, 40 min; e 1150 °C, 40 min; f 1150 °C, 120 min. Black lines represent grain boundaries with a misorientation more than 15°, and red lines represent annealing twins boundaries

Fig. 2 Austenite grain size of experimental steels with various solution treatment processes

Table 2 Mechanical properties of experimental steels

Grain size (μm)	25 °C				- 196 °C
Grain size (μm)	Yield strength (MPa)	Tensile strength (MPa)	Tensile elongation (%)	Impact energy (J)	Yield strength (MPa)	Tensile strength (MPa)	Tensile elongation (%)	Impact energy (J)
14.6	290	889	68	212	665	1300	35.5	170
21.3	274	864	71	226	655	1280	37.5	168
40.2	273	852	73	237	630	1250	36.5	172
52.7	269	821	74	252	615	1140	34	176
79.6	252	782	76.5	252	605	1130	36	179
92.4	250	766	80	264	595	1100	35.5	176

Fig. 3 Strength as a function of grain size

Fig. 4 Grain size dependence of yield strength of experimental steels at 25 °C a, - 196 °C b

Fig. 5 Engineering stress-strain curves of experimental steels at 25 °C a, - 196 °C b

Fig. 6 Tensile elongation a and impact energy b versus grain size at 25 °C, - 196 °C

Fig. 7 Fracture morphologies of tensile specimens at 25 °C a-f, - 196 °C a′-f′ for grain sizes of 14.6 μm a, a′, 21.3 μm b, b′, 40.2 μm c, c′, 52.7 μm d, d′, 79.6 μm e, e′, 92.4 μm f, f′

Fig. 8 Fracture morphologies of impact specimens at 25 °C a-f, - 196 °C a′-f′ for grain sizes of 14.6 μm a, a′, 21.3 μm b, b′, 40.2 μm c, c′, 52.7 μm d, d′ 79.6 μm e, e′, 92.4 μm f, f′

Fig. 9 XRD patterns of experimental steel with a grain size of 92.4 μm after - 196 °C impact and tensile fracture

Fig. 10 TEM images of mechanical twin in tensile fracture with a grain size of 92.4 μm at 25 °C a, - 196 °C b

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