Enhanced Welding Properties of High Strength Steel via Rare Earth Oxide Metallurgy Technology
LU Bin1,2,3, CHEN Furong1(), ZHI Jianguo2,3, GENG Ruming4
1 School of Materials Science and Engineering, Inner Mongolia University of Technology, Hohhot 010051, China 2 Inner Mongolia Baotou Steel Union Co. , Ltd. , Baotou 014010, China 3 Inner Mongolia Enterprise Key Laboratory of Rare Earth Steel Products Research and Development, Baotou 014010, China 4 State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing 100083, China
Cite this article:
LU Bin, CHEN Furong, ZHI Jianguo, GENG Ruming. Enhanced Welding Properties of High Strength Steel via Rare Earth Oxide Metallurgy Technology. Acta Metall Sin, 2020, 56(9): 1206-1216.
With the increase of the strength of steel plate, the welding performance of the steel decreases sharply and the welding crack susceptibility increases. The properties of welding heat-affected zone of high strength steel lower with increasing the welding heat input. The present work aims to improve the toughness of welding heat-affected zone by rare earth oxide metallurgy technology. The effect of 5×10-6 and 23×10-6 rare earth Ce on the impact toughness, microstructure, austenite grains of heat-affected zone and fracture morphology of welded joint were studied. When the steel contains 5×10-6 rare earth Ce, the inclusions are MgO-Al2O3 spinels surrounded with a small amount of CeAlO3 inclusions. In the case of the steel with 23×10-6Ce, Ce can completely modify MgO-Al2O3 inclusions, resulting in the formation of (CeCa)S+MgO-Al2O3+MnS complex inclusions. The simulation welding of high strength steel was performed. The results show that the Charpy impact energy of heat-affected zone of the steel with 23×10-6Ce is higher at four different heat inputs, in comparison with the steel with 5×10-6Ce. The microstructure analysis shows that the fracture morphology of heat-affected zone of the steel with 23×10-6Ce appears dimples, which is an indication of a higher toughness. With increasing the heat input from 25 kJ/cm to 100 kJ/cm, the average grain size of the original austenite in the heat-affected zones of the steels with 5×10-6Ce and 23×10-6Ce was increased by 75.6% and 52.4%, respectively. It indicates that the growth of the original austenite grain during welding is suppressed with increasing the Ce content in the steel. Comparison of the microstructure shows that rare earth Ce can delay the formation of upper bainite structure in the heat-affected zone. Through the high temperature confocal microscope, it was observed that the rare earth inclusions pinned on the original austenite grain boundary, which can effectively restrain the grain growth during the welding process. It provides an evidence showing the mechanism of improvement in the heat-affected zone in the welding of the high strength steel by rare earth Ce. The present study demonstrates the rare earth oxide metallurgy can improve the weldability of the high strength steel.
Table 1 Chemical compositions of high strength steel with different Ce contents
Fig.1 SEM image and EDS analyses of typical inclusions MgO-Al2O3+(CaMn)S+CeAlO3 in 5Ce steel
Steel
ReL / MPa
Rm / MPa
A / %
AKV / J
5Ce
782
812
15.5
148
23Ce
794
838
17.0
212
Table 2 Mechanical properties of high strength steel with different Ce contents
Fig.2 SEM image and EDS analyses of typical inclusions (CeCa)S+MgO-Al2O3+MnS in 23Ce steel
Heat input
kJ·cm-1
Peak temp.
℃
Holding time
s
t8/5
s
Cooling rate
℃·s-1
25
1350
1
18
16.67
50
1350
1
74
4.05
75
1350
1
165
1.82
100
1350
1
295
1.02
Table 3 Parameters of simulated welding thermal cycle
Fig.3 Thermal simulation process of heat-affacted zone (HAZ)
Fig.4 Impact energies of HAZ under different heat inputs at room temperature
Fig.5 OM images of HAZ in 5Ce steel under heat inputs of 25 kJ/cm (a), 50 kJ/cm (b), 75 kJ/cm (c) and 100 kJ/cm (d)
Fig.6 OM images of HAZ in 23Ce steel under heat inputs of 25 kJ/cm (a), 50 kJ/cm (b), 75 kJ/cm (c) and 100 kJ/cm (d)
Fig.7 SEM fractographs of 5Ce steel under heat inputs of 25 kJ/cm (a), 50 kJ/cm (b), 75 kJ/cm (c) and 100 kJ/cm (d)
Fig.8 SEM fractographs of 23Ce steel under heat inputs of 25 kJ/cm (a), 50 kJ/cm (b), 75 kJ/cm (c) and 100 kJ/cm (d)
Fig.9 OM images of HAZ original austenite grain in 5Ce steel under heat inputs of 25 kJ/cm (a), 50 kJ/cm (b), 75 kJ/cm (c) and 100 kJ/cm (d)
Steel
25 kJ·cm-1
50 kJ·cm-1
75 kJ·cm-1
100 kJ·cm-1
5Ce
40.2
59.9
66.9
70.6
23Ce
47.3
49.3
50.2
72.1
Table 4 Grain sizes of original austenite in HAZ under different heat inputs
Fig.10 OM images of HAZ original austenite grain in 23Ce steel under heat inputs of 25 kJ/cm (a), 50 kJ/cm (b), 75 kJ/cm (c) and 100 kJ/cm (d)
Fig.11 High temperature confocal observation results of 23Ce steel (Original grain bounaries (dashed lines) move in the direction of arrows)
Temperature / ℃
25 kJ·cm-1
50 kJ·cm-1
75 kJ·cm-1
100 kJ·cm-1
>930
8.51
24.93
52.07
90.93
>1000
6.97
19.57
41.11
70.57
>1100
5.10
14.08
28.22
48.08
>1200
3.46
9.08
17.72
30.58
>1300
1.86
3.58
8.22
14.08
Table 5 Holding time of 23Ce steel at different temperatures under different heat inputs
Fig.12 SEM images and EDS analyses of inclusions in 23Ce steel after electrolysis
Fig.13 SEM images and EDS of carbonitride in 23Ce steel after electrolysis
Fig.14 SEM images and EDS analyses of Ce-contained inclusions in 23Ce steel after electrolysis
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