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Acta Metall Sin  2017, Vol. 53 Issue (5): 549-558    DOI: 10.11900/0412.1961.2016.00316
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Effect of Surface Layer with Ultrafine Grains on Crack Arrestability of Heavy Plate
Jiangnan MA1,Ruizhen WANG2(),Caifu YANG2,Xiaoqin ZHA1,Lijuan ZHANG1
1 Luoyang Ship Material Research Institute, Luoyang 471023, China
2 Department of Engineering Steels, Central Iron and Steel Research Institute, Beijing 100081, China
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Abstract  

Temperature reverting rolling process (TRRP) is a newly developed technology for producing heavy steel plate with ultrafine grained surface layer. With hybrid structures along thickness direction, TRRP steel plate has excellent fracture toughness with crack arrestability which arouses interest recently. However, the crack arrest mechanism of the surface layer is still unclear to date. In this work, two types of steel plate produced by TRRP and traditional thermo mechanical control process (TMCP) were studied in order to get a comprehensive understanding of the crack arrest mechanism. The mechanical property tests demonstrate that the toughness of surface layer of TRRP steel is significantly higher than that of TMCP steel, while the mechanical properties at 1/4 thickness position of the two types are quite close. It's worth noting that ductile-brittle transition temperature of the TRRP steel surface layer is as low as -100 ℃. Microstructure analysis of the TRRP surface layer shows a coexistence of equiaxed ferrite grains with grain sizes of about 2 μm and dispersed M-A constituent. Numerical simulation of the temperature field of TRRP intermediate slab reveals the microstructure forming process. First, the surface layer is cooled lower than phase transformation temperature, which results in the generation of bainite ferrite. Subsequently, dynamic recrystallization of ferrite takes place in rolling process and leads to the formation of ultrafine grains. Instrumental impact test at -60 ℃ shows that the crack propagation of TRRP steel is effectively inhibited after a steady developing stage. The morphological analysis of the cross section of fracture shows significant plastic deformation in the surface layer, which means crack propagation energy is absorbed. As a result, the crack propagation is efficiently arrested. The statistical study of the grain orientations in the surface layer of TRRP steel indicates a randomly distribution of the ultrafine grains, which can hinder the crack propagation effectively. The nano indentation test shows that the hardness distributions of TRRP steel are mainly below 2.0 GPa. This means the microstructure is characterized by a small amount of hard phase dispersing in soft matrix, thus the crack initiated at the interface of phases can hardly propagate.

Key words:  heavy plate      temperature-reverting rolling process      surface layer with ultrafine grain      crack arrest      instrumental impact     
Received:  20 July 2016     

Cite this article: 

Jiangnan MA,Ruizhen WANG,Caifu YANG,Xiaoqin ZHA,Lijuan ZHANG. Effect of Surface Layer with Ultrafine Grains on Crack Arrestability of Heavy Plate. Acta Metall Sin, 2017, 53(5): 549-558.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00316     OR     https://www.ams.org.cn/EN/Y2017/V53/I5/549

Fig.1  OM images of temperature reverting rolling process (TRRP) steel((a) macrostructure (b) microstructure of region I in Fig.1a(c) microstructure of region II in Fig.1a (d) microstructure of region III in Fig.1a)
Fig.2  SEM images of surface layer (a, c) and t/4 (b, d) of TRRP (a, b) and TMCP (c, d) steels (UF—ultrafine ferrite, M-A—martensite-austenite, AF—acicular ferrite, QPF—quasipolygonal ferrite, UB—upper bainite, PF—polygonal ferrite, TMCP—thermo mechanical control process, t—thickness)
Steel Rm / MPa Rel / MPa A / %
Surface t/4 Surface t/4 Surface t/4
TRRP 621 600 545 520 21.0 25.5
TMCP 652 610 609 545 13.0 25.5
Table1  Tensile properties of surface layer and t/4 of TRRP and TMCP steels
Fig.3  Charpy absorbed energy vs temperature of TRRP (a) and TMCP (b) steels
Fig.4  Phase transformation behavior of microstructure through thickness of TRRP steel intermediate slab during water cooling and reverting (SUF—surface layer with ultrafine-grained microstructure, A—austenite, B—bainite, F—ferrite, P—pearlite, Ac1—austenite transformation beginning temperature during calefaction, Ac3—austenite transformation finish temperature during calefaction, Tuf—critical temperature for ultra-fine grain forming)
Fig.5  Temperature distribution along thickness of TRRP steel intermediate slab at the end of water cooling
Fig.6  Curves of load and absorbed energy vs hammer displacement at -60 ℃ (E1—crack initiation absorbed energy, E2—crack steady propagation absorbed energy, E3—crack arrest/collapse absorbed energy, E4—post crack arrest/collapse absorbed energy, Pm—max load, Pf—load at the beginning of crack arrest/collapse, Pa—load at the end of crack arrest/collapse)((a) surface layer of TRRP steel (b) surface layer of TMCP steel (c) t/2 of TMCP steel)
Fig.7  Fractographs of instrumented impact samples (Figs.7b, e and h are magnified images of marked area in Figs.7a, d and g. Figs.7c, f and i are cross section images of fracture)((a~c) surface layer of TRRP steel (d~f) surface layer of TMCP steel (g~i) t/2 of TMCP steel)
Fig.8  Inverse pole figure (IPF) maps of surface layer of TRRP (a) and TMCP (b) steels
Fig.9  Orientation distribution function (ODF) maps of surface layer of TRRP (a) and TMCP (b) steels (Φ, φ1, φ2—Euler angles)
Fig.10  Misorientation maps (a, c) and misorientation angle distribution diagrams (b, d) of surface layer of TRRP (a, b) and TMCP (c, d) steels (In Figs.10a and c, black lines show grain boundaries larger than 15°, red lines show grain boundaries between 2°and 15°)
Fig.11  SEM images of nanoindentations of surface layer of TRRP (a) and TMCP (b) steels
Fig.12  Statistics of the nanoindentation hardness of surface layer of TRRP (a) and TMCP (b) steels
Fig.13  Secondary crack in cross section of Charpy impacted fracture at -140 ℃ of TMCP steel
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