Control of Secondary Phases by Solution Treatment in a N-Alloyed High-Mn Cryogenic Steel

doi:10.1007/s40195-018-0759-8

Abstract

Abstract:

The secondary phases of the steels have significant effects on the microstructure and mechanical properties, making controlling these secondary phases important. The control of MnS inclusions and AlN precipitates in a N-alloyed high-Mn twin-induced plastic cryogenic steel via solution treatment was investigated with several different techniques including microstructural characterization, 298 K tensile testing, and 77 K impact testing. The solutionizing temperature (ST) increased from 1323 to 1573 K, where the elongated MnS inclusions and large-sized AlN precipitates became spheroidized and dissolved. The aspect ratio of the MnS inclusions decreased as the ST increased and the number density increased. The impact toughness of the steels showed anisotropy and low impact energy values, due to the elongated MnS inclusions and large-sized AIN precipitates. The anisotropy was eliminated by spheroidizing the MnS inclusions. The impact energy was improved by dissolving the large-sized AlN precipitates during the solution treatment. The austenite grain size increased when the dissolution of the AlN precipitate increased, but the effect of the grain size on the yield strength, toughness, and the strength-ductility balance was weak.

Key words: Cryogenic, Solutionizing temperature, Low-temperature toughness, Spheroidize, Dissolution

Xiao-Jiang Wang, Xin-Jun Sun, Cheng Song, Shuai Tong, Luo-Jin Liu, Huan Chen, Wei Han, Feng Pan. Control of Secondary Phases by Solution Treatment in a N-Alloyed High-Mn Cryogenic Steel[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(10): 1059-1072.

Figures/Tables 16

Table 1 Chemical compositions of the materials (wt%)

C	Si	Mn	S	P	Cr	Mo	N	Al
0.48	0.21	24.28	0.0091	0.0059	6.29	0.32	0.19	0.024

Fig. 1 Rolling and solution treatment processes a and in situ observation experiment process b

Fig. 2 Microstructure, MnS inclusion, and AlN precipitate of the hot-rolled steel. a OM image (the red arrow refers to MnS inclusion), b SEM image of MnS inclusion and AlN precipitate, c, d are the EDS spectra of MnS inclusion and AlN precipitate shown in b

Fig. 3 SEM images of MnS inclusions and AlN precipitates in the different specimens with various STs. a 1323 K, b 1373 K, c 1423 K, d 1473 K, e 1523 K, f 1573 K

Fig. 4 Average sizes a, aspect ratios b, and number densities c of MnS inclusion with various STs

Fig. 5 CSLM images (a 297.6 K, b 1323 K, c 1373 K, d 1423 K, e 1473 K, f 1523 K, g 1573 K, h 1573 K?+?2 min) and the images of SEM (include EDS spectrum) i of MnS inclusion after cooling down

Fig. 6 Morphologies (large size a and small size b), selected area detection diffraction pattern c and EDS d of AlN precipitate in the steel with 1323 K solution treatment. The volume fraction of AlN precipitates versus temperature curve calculated by Thermo-Calc software e

Table 2 Mechanical properties of the hot-rolled steels with various directions

Angle (°)	Yield strength (MPa)	Tensile strength (MPa)	Elongation (%)	Impact energy (J)
0	438	954	67	98
45	437	941	68	72
90	440	948	68	53

Fig. 7 Tensile strength, yield strength and impact energy of the hot-rolled steels along various directions

Fig. 8 77 K impact fracture morphologies of hot-rolled steels along various directions. a 0°, b 45°, c 90° and d EDS of MnS inclusions. The black arrow refers to MnS inclusion

Fig. 9 Impact energies of experimental steels along different directions with various STs a and the different percentages of the impact toughness along various directions b

Fig. 10 Impact fractures of the specimens at 0° direction in the experimental steels with various STs (red circles represent AlN precipitates). a 1323 K, EDS of AlN precipitates, b 1373 K, c 1423 K, d 1473 K, e 1523 K, f 1573 K

Fig. 11 EBSD images of the austenite grains with various ST. a 1323 K, b 1373 K, c 1423 K, d 1473 K, e 1523 K, f 1573 K. Black lines represent effective grains with a misorientation more than 15°, red lines represent annealing twins

Fig. 12 Austenite grain size as a function of ST

Table 3 Tensile properties of the experimental steels with various ST

Solutionizing temperature (K)	Yield strength (MPa)	Tensile strength (MPa)	Elongation (%)	Strength-ductility balance (GPa%)
1323	423	906	68	61.6
1373	414	864	70	60.5
1423	409	837	71	59.4
1473	402	824	74	61.0
1523	400	810	76	61.5
1573	397	783	79	61.9

Fig. 13 Tensile properties of the experimental steels with various STs. a Engineering stress versus engineering strain curves, b true stress and strain hardening rate versus true strain curves, c tensile and yield strength as a function of ST, d grain size dependence of the yield strength of experimental steels

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