EFFECTS OF O, N AND Ni CONTENTS ON HOT PLASTICITY OF 0Cr25Ni7Mo4N DUPLEX STAINLESS STEEL
CHEN Yulai1, ZHANG Tairan1, WANG Yide2, LI Jingyuan2()
1 Metallurgical Engineering Research Institute, University of Science and Technology Beijing, Beijing 100083 2 School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083
Cite this article:
CHEN Yulai, ZHANG Tairan, WANG Yide, LI Jingyuan. EFFECTS OF O, N AND Ni CONTENTS ON HOT PLASTICITY OF 0Cr25Ni7Mo4N DUPLEX STAINLESS STEEL. Acta Metall Sin, 2014, 50(8): 905-912.
In order to identity the effect of narrow composition control on the hot plasticity of duplex stainless steel, hot rolling test of OCr25Ni7Mo4N steels with various oxygen, nitrogen and nickel contents were performed at 1200 ℃ for 4 steps. The microstructures and inclusions were observed by OM, SEM and EBSD. The steels with the lowest oxygen, nitrogen and nickel contents showed excellent hot plasticity. The inclusions in the steel with 0.0059% oxygen were mainly Al2O3 and MgO·Al2O3, which distributed in the grain interior and did no harm to the hot plasticity of the steel. The steels containing 0.038% and 0.046% oxygen actually cracked at the sheet edge during hot rolling, which resulted from the large inclusion particles of Cr2O3 and MnO2 at the α/γ boundary. Furthermore, the reason for more serious cracking occurred in the steel containing 0.038% oxygen than that containing 0.046% oxygen was its relatively higher contents of nitrogen and nickel, making γ volume fraction of the steel as high as 60% in the hot rolling state. Excessive γ reduced its total strain, so that the inadequate stain did not induce the recrystallization of γ phase, which resulted in hot rolling cracking finally.
Fund: Supported by National Natural Science Foundation of China (No.51174026), National Science and Technology Pillar Program during the Twelfth Five-Year Plan Period (No.2012BA-E04B02)
Table 1 Chemical compositions of tested steels and SAF 2507 steel
Fig.1 Morphologies of 0Cr25Ni7Mo4N duplex stainless steel sheets after hot rolling at 1200 ℃ for 4 steps
Fig.2 Effect of γ/α phase proportion on hot plasticity of duplex stainless steel[1]
Steel
Cast
Before rolled
After rolled
No.1
41.47
49.02
37.55
No.2
29.51
36.23
27.93
No.3
40.53
45.39
32.01
Table 2 Ferrite contents of tested 0Cr25Ni7Mo4N duplex stainless steels at different processes
Fig.3 OM images of No.1 (a, d), No.2 (b, e) and No.3 (c, f) 0Cr25Ni7Mo4N duplex stainless steels at casting state (a~c) and before hot rolling (d~e) (Inset in Fig.3d shows the high magnified image of α, γ and Cr2N)
Fig.4 High contrast SEM images of inclusions in tested steels No.1 (a), No.2 (b) and No.3 (c)
Steel
Fraction of inclusions with different sizes / %
Average
Total
diameter / μm
Number / mm-2
0~5 μm
5~10 μm
10~15 μm
>15 μm
No.1
84.21
15.79
0
0
4.04
197
No.2
17.86
21.45
49.98
10.71
11.02
554
No.3
18.02
27.03
41.44
13.51
10.68
1034
Table 3 Sizes and amounts of inclusions in tested steels
Fig.5 SEM images (left) and corresponding EDS analyses (right) of inclusions in steels No.1 (a, b), No.2 (c) and No.3 (d)
Fig.6 Location of inclusions in steels No.1 (a) and No.2 (b)
Fig.7 Morphology (left) and EDS analysis (right) of the inclusions at the crack area of No.2 hot rolled sheet
Steel
Cast
Before Rolled
No.1
300.6
301.6
No.2
291.3
303.9
No.3
298.0
305.7
Table 4 Microhardness of γ phase in tested steels
Fig.8 EBSD morphologies and deformation distributions at the side of 0Cr25Ni7Mo4N duplex stainless steel sheets
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