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Acta Metall Sin  2018, Vol. 54 Issue (10): 1387-1398    DOI: 10.11900/0412.1961.2018.00100
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Microstructure and Mechanical Properties of an Austenite/Ferrite Laminate Structured High-Manganese Steel
Kai ZHU, Cuilan WU(), Pan XIE, Mei HAN, Yuanrui LIU, Xiangge ZHANG, Jianghua CHEN
College of Materials Science and Engineering, Hunan University, Changsha 410082, China
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Abstract  

Lamianate structured metals have recently attracted extensive interests for their outstanding mechanical properties which are produced by synergetic strengthening of different heterogeneous layers. For Mn-rich maraging steels, austenite can precipitate in the martensite matrix due to Mn segregation during heat treatment, and then the austenite and martensite/ferrite duplex steel is produced. Besides, high Mn TRIP steel is regarded as a promising material for next generation automobile steel because of its high strength and good ductility. In this work, the microstructures and properties of Mn12Ni2MoTi(Al) steels produced by thermo-mechanical process were investigated using XRD, SEM, TEM, EBSD, EPMA, hardness tests and tensile tests. The results showed that laminate structures with austenite/ferrite band alternate arranging along the normal direction were formed in Mn12Ni2MoTi(Al) steels, which were processed by 65% cold-rolling and subsequent annealing at 745 ℃. Both of austenite bands and ferrite bands consist of ultrafine equiaxed grains. Moreover, a small amount of ferrite grains and austenite grains were found inside the austenite bands and ferrite bands, respectively. In the austenite/ferrite laminate structures, the austenite bands show the Brass texture of {110}<112> and Goss- type texture of {110}<001>. The ferrite bands show the rotate Cube texture of {001} <110> and Cube texture of {001}<100>. With the increase of annealing time, the laminate structures first become dominant and then disappear gradually, accompanying with the orientation transition of austenite from the Brass texture into the Goss one. The samples with laminate structures have high yield strength and good ductility. Otherwise, when the laminate structures disappear, the yield strength and ductility of the samples will decrease, and the yield strength deceases more.

Key words:  laminate structure      high manganese steel      thermomechanical process      TRIP effect     
Received:  19 March 2018     
ZTFLH:  TG166.7  
Fund: Supported by National Natural Science Foundation of China (Nos.11427806 and 51371081)

Cite this article: 

Kai ZHU, Cuilan WU, Pan XIE, Mei HAN, Yuanrui LIU, Xiangge ZHANG, Jianghua CHEN. Microstructure and Mechanical Properties of an Austenite/Ferrite Laminate Structured High-Manganese Steel. Acta Metall Sin, 2018, 54(10): 1387-1398.

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https://www.ams.org.cn/EN/10.11900/0412.1961.2018.00100     OR     https://www.ams.org.cn/EN/Y2018/V54/I10/1387

Fig.1  Schematic of a sample (ND-section, TD-section and RD-section are perpendicular to the normal direction, the transverse direction and the rolling direction, respectively)
Texture component Symbol Miller index
Cube {001}<100>
Rotated Cube {001}<110>
Goss {110}<001>
Brass {110}<112>
E-typed {111}<110>
F-typed {111}<112>
Copper {112}<111>
Rotated Goss {110}<110>
Table 1  The Miller indices and texture components of symbols given in Fig.2
Fig.2  Locations of common textures components of metals in the ψ2=0° and ψ2=45° sections (The Miller indices and texture components of symbols are given in Table 1)[29]
Fig.3  TEM image (a), ND-sectional XRD spectrum (b) and ODFs of texture at ψ2=0° (c) and ψ2=45° (d) of 65%CR sample
Fig.4  SEM images of 745 ℃-1 h sample in different sections
(a) ND-section (b) TD-section (c) RD-section (d) low magnified image of the TD-section
Fig.5  The average thickness curves (a) and Vickers hardness curves (b) of bright bands and dark bands in the same samples annealed for different time at 745 ℃ (Inset shows the indentations of different bands)
Fig.6  SEM image (a) and EBSD phase-map (b) of the 745 ℃-1 h sample
Fig.7  XRD spectra of 745 ℃-1 h annealed samples in different sections
Fig.8  The relative volume fraction of γ in the samples annealed at 745 ℃ for different time
Fig.9  EBSD phase-maps (a~f) and IPF-maps (a1~f1) of the samples annealed at 745 ℃ for different time (In the EBSD phase-maps, the red represents the austenite while the green represents the ferrite. In the IPF-maps, different colours represent different directions of grains)
(a, a1) 745 ℃-5 min (b, b1) 745 ℃-30 min (c, c1, e, e1, f, f1) 745 ℃-1 h (d, d1) 745 ℃-24 h
Fig.10  Textures of austenite in the different samples annealed for different time at 745 ℃
(a, a1) 745 ℃-5 min (b, b1) 745 ℃-30 min (c, c1) 745 ℃-1 h (d, d1) 745 ℃-24 h
Fig.11  Textures of ferrite in the different samples annealed for different time at 745 ℃
(a, a1) 745 ℃-5 min (b, b1) 745 ℃-30 min (c, c1) 745 ℃-1 h (d, d1) 745 ℃-24 h
Fig.12  EPMA maps of Mn distribution for different samples
(a) 65%CR (b) 745 ℃-30 min (c) 745 ℃-24 h
Fig.13  TEM images of 745 ℃-1 h (a, c) and 745 ℃-24 h (b, d) samples (a, b) recrystallized grains (c, d) martensite
Fig.14  Tensile stress-strain curves of different samples
Fig.15  EBSD phase-maps (a~d) and IPF-maps (a1~d1) of the 745 ℃-1 h sample with different tensile deformations (In the EBSD phase-maps, the red represents the austenite while the green represents the ferrite. In the IPF-maps, different colours represent different directions of grains)
(a, a1) 0.5% strain (b, b1) 5% strain (c, c1) 10% strain (d, d1) tensile fracture
Fig.16  EBSD phase-map (a) and IPF-map (a1) of the 745 ℃-24 h sample after tensile fracture (In the EBSD phase-maps, the red represents the austenite while the green represents the ferrite. In the IPF-maps, different colours represent different directions of grains)
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