Recent progress in medium-Mn steels made with new designing strategies, a review

doi:10.1016/j.jmst.2017.06.017

Abstract

Abstract:

After summarizing the relevant researches on the medium Mn steels in references, two new targets on the tensile properties have been defined. One is that both transformation-induced (TRIP) and twinning-induced plasticity (TWIP) could be realized for the steel with a relatively low Mn content, which exhibits the similar tensile properties to the classical TWIP steels with higher Mn content. The other is to achieve ultrahigh ultimate tensile strength (>1.5 GPa) without sacrificing formability. To achieve these goals, new designing strategies was put forward for compositions and the processing route. In particular, warm rolling was employed instead of the usual hot/cold rolling process because the former can produce a mixture of retained austenite grains with different morphologies and sizes via the partial recrystallization. Consequently, the retained austenite grains have a wide range of mechanic stability so that they can transform to martensite gradually during deformation, leading to enhanced TRIP effect and then improved mechanic properties. Finally, it is succeeded in manufacturing these targeted medium Mn steels in laboratory, some of them even exhibit better tensile properties than our expectation.

Key words: Medium Mn steel, Retained austenite, Transformation-induced plasticity, Twinning-induced plasticity, Mechanical properties

Hu Bin, Luo Haiwen, Yang Feng, Dong Han. Recent progress in medium-Mn steels made with new designing strategies, a review[J]. J. Mater. Sci. Technol., 2017, 33(12): 1457-1464.

Figures/Tables 9

Fig. 1. Summary of tensile strength and tensile elongation data for various classes of conventional and advanced high strength sheet steels (AHSS) [1,2].

Fig. 2. Summary of ultimate tensile strength (UTS) and total elongation (TE) (a), and the products of UTS and TE (b) for the medium Mn steels that have been published in references. The new targets have been defined in the inset.

Table 1 Main chemical compositions of different medium manganese steels.

C (wt%)	Mn (wt%)	Si (wt%)	Al (wt%)	Mo (wt%)	V (wt%)	RA (%)	Ref.
0.092	4.60	0.03				22.0	[25]
0.120	4.98	3.11	3.05	0.05		15.2	[3]
0.190	4.96	3.09	2.99	0.03		9.0	[3]
0.200	5.00					34.1	[10]
0.400	5.00					40.0	[10]
0.120	5.80	0.47	3.00			31.0	[12]
0.050	6.15	1.50				11.0	[26]
0.080	6.15	1.50	2.00		0.08	17.0	[27]
0.200	7.00					44.7	[13]
0.099	7.09	0.13	0.03			43.5	[13]
0.220	7.15	3.11	3.21	0.05		13.0	[3]
0.230	8.10	0.01	5.30			53.0	[15]
0.260	10.00		6.30			45.0	[14]
0.200	10.02	3.17	3.19	0.06		53.0	[3]
0.300	10.00	2.00	3.00			67.0	[21]
0.200	11.00		1.40			68.5	[28]
0.180	11.02		3.81			66.0	[29]
0.600	12.00						[30]
0.200	12.40	0.90	5.20			71.0	[15]

Fig. 3. Engineering stress-strain curves of developed 10%Mn-0.7%V steels after warm rolling and intercritical annealing. A, B, C, D represent the tensile properties of studied steels that have been warm rolled at different temperatures with various rolling reductions in thickness. A: 750 °C, 50%; B: 750 °C, 63%; C: 600 °C, 50%; D: 600 °C, 63%.

Fig. 4. Calculated equilibrium phase fractions (a) and the composition of austenite (b) for the developed 7 Mn steel; (c) the engineering stress-strain curve of 7 Mn steel sheet, which was manufactured by hot rolling, warm rolling and the intercritical annealing at 700 °C for 5 h; (d) XRD patterns of studied 7 Mn sample before deformation and after fracture, which confirms the significant TRIP effect during deformation due to the large decrease of retained austenite fraction; (e) mechanical twins observed on the fractured specimen, indicating that TWIP effect can occur during deformation.

Fig. 5. Warm rolling process used to manufacture 10 Mn and 7 Mn steels (a); XRD patterns of 7 Mn steel after hot rolling (b) and after warm rolling at 600 °C (c).

Fig. 6. Typical microstructures of 7 Mn steel after warm rolling at 600 °C (a) and after the intercritical annealing at 700 °C for 5 h (b); the magnification views of regions ‘A’ and ‘B’ in (b) are given in (c) and (d) respectively, in which γG and γL denote the globular and the lath-like austenite grains and their corresponding selected area electron diffraction patterns are given in the insets.

Fig. 7. Tensile properties of medium Mn steels recently developed in our laboratory, some of them are better than those reported [46-49].

Fig. 8. Schematic diagram of elements partition during intercritical annealing and the deformation mechanism under different periods.

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