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Acta Metall Sin  2020, Vol. 56 Issue (9): 1227-1238    DOI: 10.11900/0412.1961.2020.00007
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Cross-Section Effect of Ni-Cr-Mo-B Ultra-Heavy Steel Plate for Offshore Platform
ZHANG Shouqing1,2, HU Xiaofeng1, DU Yubin1,2, JIANG Haichang1, PANG Huiyong3, RONG Lijian1()
1 CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
2 School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
3 Wuyang Iron and Steel Co. Ltd. , Pingdingshan 462500, China
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Abstract  

With the increasing demand and exploitation depth for offshore oil and gas, offshore platforms are becoming larger and the performance requirements and size for offshore platform of ultra-heavy plates are also increasing. Due to the large plate thickness and the limitation of manufacturing techniques, inhomogeneous microstructures and mechanical properties along thickness direction are great challenges for offshore platform of ultra-heavy plates. In this work, variation of microstructure and its effect on mechanical properties for the 117 mm-thick Ni-Cr-Mo-B industrial ultra-heavy plate were investigated by means of OM, SEM, TEM and EBSD observation, in combination with the tensile and impact toughness test. The results show that yield strength reduces gradually from the surface (798 MPa) to the center (718 MPa) and elongation almost keeps constant around 20.0%~22.0% for the 117 mm-thick plate. It is noted that impact energy at -60 ℃ increases first from 35 J at the surface and reaches its peak 160 J at the depth of 1/8T (T—thickness of plate), and then drops to the minimum about 20 J at the center, which suggests that impact energy curve along the whole section varies sharply and exhibits like letter 'M'. Lath width, boundary carbide size and intragranular carbide size are all gradually increasing from the surface to the center, i.e., from 198.7 nm to 500.6 nm, 130.6 nm to 226.6 nm, 45.8 nm to 106.2 nm, respectively, and there are also some blocky areas at the center, all those indicate that refinement strengthening and precipitation strengthening would decrease, as well as the gradual decrease of yield strength. Also, from the surface to the center, effective grain size (EGS) decreases first and then increases. The surface and the center have larger EGS (2.2 μm and 2.7 μm, respectively), which indicates that they have weaker resistance to cleavage crack and exhibit lower impact energy. However, the 1/8T position has smaller EGS (1.7 μm) while obtains higher impact energy.

Key words:  Ni-Cr-Mo-B steel      ultra-heavy plate      cross-section effect      impact energy      effective grain size     
Received:  07 January 2020     
ZTFLH:  TG142.1  
Fund: National Key Research and Development Program of China(2016YFB0300601);Liaoning Revitalization Talents Program(XLYC1907143);Major Science and Technology Projects of Construction Corps(2017AA004-2)
Corresponding Authors:  RONG Lijian     E-mail:  ljrong@imr.ac.cn

Cite this article: 

ZHANG Shouqing, HU Xiaofeng, DU Yubin, JIANG Haichang, PANG Huiyong, RONG Lijian. Cross-Section Effect of Ni-Cr-Mo-B Ultra-Heavy Steel Plate for Offshore Platform. Acta Metall Sin, 2020, 56(9): 1227-1238.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2020.00007     OR     https://www.ams.org.cn/EN/Y2020/V56/I9/1227

Fig.1  Schematics of samples for mechanical property tests and microstructure observations (a), and impact fracture cross-section (b) for the Ni-Cr-Mo-B ultra-heavy plate (RD—rolling direction, TD—transverse direction, ND—normal direction, T—thickness of Ni-Cr-Mo-B ultra-heavy plate)
PositionCNiMnMoCrBSiNbVTiCuAlSPFe
Surface0.151.350.950.441.030.00070.170.0250.0450.020.0230.0350.0030.015Bal.
Center0.131.260.950.411.020.00110.170.0220.0410.020.0220.0310.0030.014Bal.
Table 1  Chemical compositions of the Ni-Cr-Mo-B ultra-heavy plate at the surface and the center
PositionType AType BType CType DType DS
CoarseFineCoarseFineCoarseFineCoarseFine
0T0000000.51.00
1/4T0000001.01.00
1/2T0000000.51.00.5
3/4T0000000.51.00
1T0000001.01.00
Table 2  Inclusion grade of the Ni-Cr-Mo-B ultra-heavy plate at different positions
Fig.2  Yield strength and elongation at room temperature (a), and impact energy at -60 ℃ (b) of the Ni-Cr-Mo-B ultra-heavy steel plate at different positions
Fig.3  Prior austenite grain images of the Ni-Cr-Mo-B ultra-heavy plate at 6 mm (a), 19 mm (b) and 58 mm (c)
Fig.4  OM images of the Ni-Cr-Mo-B ultra-heavy plate at different positions (LMT—tempered lath martensite, LBT—tempered lath bainite, GBT—tempered granular bainite)
Fig.5  SEM images of the Ni-Cr-Mo-B ultra-heavy plate at 6 mm (a, d), 19 mm (b, e) and 58 mm (c, f, g) (Figs.5d, e and f are magnified images of the zones marked by solid lines in Figs.5a, b and c, respectively. Fig.5g is magnified image of the zone marked by dotted lines in Fig.5c)
Fig.6  Microstructure percentage on the section of the Ni-Cr-Mo-B ultra-heavy steel plate
Fig.7  Grain boundary distribution maps for the Ni-Cr-Mo-B ultra-heavy plate at different positions (The black and red lines indicate the boundaries with misorientations of higher than 15° and 2°~15°, respectively)
Fig.8  Effective grain sizes of the Ni-Cr-Mo-B ultra-heavy steel plate at different positions
Fig.9  TEM images of the Ni-Cr-Mo-B ultra-heavy plate at 6 mm (a, e), 19 mm (b, f) and 58 mm (c, d, g) (Insets in Figs.3e~g show corresponding SAED patterns)
Fig.10  Grain boundary distribution map of impact fracture cross-section of the Ni-Cr-Mo-B ultra-heavy plate at 1/8T (The black and red lines indicate the boundaries with misorientations of higher than 15° and 2°~15°, respectively)
Fig.11  SEM images of impact specimens of the Ni-Cr-Mo-B ultra-heavy plate at 0T (a, d), 1/8T (b, e) and 1/2T (c, f) (The zones marked by dotted lines are cleavage areas. Figs.11d~f are magnified images of the zones marked by solid lines in Figs.11a~c, respectively)
Fig.12  SEM images of impact fracture cross-section of the Ni-Cr-Mo-B ultra-heavy plate at 0T (a), 1/8T (b) and 1/2T (c) (UPCL—unit primary crack length)
Fig.13  Relationships among effective grain size (EGS), unit primary crack length and impact energy at -60 ℃ (AKV) of the Ni-Cr-Mo-B ultra-heavy plate at different positions
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