Carbide precipitation behavior and mechanical properties of micro-alloyed medium Mn steel

doi:10.1016/j.jmst.2019.12.024

Abstract

Abstract:

The carbide precipitation behavior and mechanical properties of advanced high strength steel deformed at different temperatures are investigated by X-ray diffractometer (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) equipped with an energy dispersing spectroscopy (EDS), and tensile tests. The medium Mn steel was subjected to controlled deformation up to 70% at 750 °C, 850 °C, 950 °C, and 1050 °C, and then quenched with water to room temperature, followed by intercritical annealing at 630 °C for 10 min. In comparison with the undeformed and quenched specimen, it can be concluded that acicular cementite precipitates during the quenching and cooling process, while granular NbC is the deformation induced precipitate and grows during the following annealing process. As the deformation temperature increases from 750 °C to 1050 °C, the product of strength and elongation increases at first and then decreases. The smallest average size of second phase particles (20 nm) and the best mechanical properties (32.5 GPa.%) can be obtained at the deformation temperature of 950 °C.

Key words: Advanced high strength steel, Medium Mn steel, Thermal deformation, Intercritical annealing, The product of strength and elongation

Luhan Hao, Xiang Ji, Guangqian Zhang, Wei Zhao, Mingyue Sun, Yan Peng. Carbide precipitation behavior and mechanical properties of micro-alloyed medium Mn steel[J]. J. Mater. Sci. Technol., 2020, 47: 122-130.

Figures/Tables 12

Table 1 Chemical compositions of the experimental steel (mass %).

C	Mn	Si	Mo	Nb	V	P	S	Fe
0.15	4.8	1.2	0.11	0.05	0.05	0.004	0.002	Bal.

Fig. 1. Microstructure of the studied steel at hot forging state under SEM.

Fig. 2. Schematic diagram of experiment setup and positions of the studied specimen (a) and thermal deformation process (b). DT: deformation temperature; Min: minutes; S: seconds; WQ: water quenching; AC: air cooling.

Fig. 3. SEM microstructure of the experimental steel at quenched state after deformation at (a) 750 °C; (b) 850 °C; (c) 950 °C; (d) 1050 °C.

Fig. 4. SEM microstructure of the intercritical annealed specimens after deformation at (a) 750 °C; (b) 850 °C; (c) 950 °C; (d) 1050 °C.

Fig. 5. TEM microstructure and substructure of the annealed specimens after deformation at (a) 750 °C; (b) 850 °C; (c) 950 °C; (d) 1050 °C.

Fig. 6. TEM images with SAD pattern of M3C (a) and EDS analysis of the precipitates (b) of the quenched specimen without deformation.

Fig. 7. TEM images of precipitates in quenched specimens after 70% deformation at (a) 750 °C; (b) 850 °C; (c) 950 °C; (d) 1050 °C.

Fig. 8. TEM images of precipitates of the annealed specimens after 70% deformation at (a) 750 °C; (b) 850 °C; (c) 950 °C; (d) 1050 °C and EDS analysis of precipitates and matrix of area 1 (e), area 2 (f) and area 3 (g).

Fig. 9. X-ray diffraction pattern of precipitates extracted from the annealed specimens after 70% deformation at 750 °C, 850 °C, 950 °C, and 1050 °C.

Fig. 10. Particle size distribution of the precipitates in the annealed specimens.

Fig. 11. The strength (a) and plasticity (b) of the experimental steel at the quenched and annealed state after deformation.

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