EFFECT OF FINISH COOLING TEMPERATURE ON MICROSTRUCTURE AND LOW TEMPERATURE TOUGHNESS OF Mn-SERIES ULTRA-LOW CARBON HIGH STRENGTH LOW ALLOYED STEEL
GAO Guhui1, GUI Xiaolu1, AN Baifeng1, TAN Zhunli1, BAI Bingzhe2(), WENG Yuqing2
1 Material Science and Engineering Research Center, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044 2 Key Laboratory of Advanced Material, School of Materials Science and Engineering, Tsinghua University, Beijing 100084
Cite this article:
GAO Guhui, GUI Xiaolu, AN Baifeng, TAN Zhunli, BAI Bingzhe, WENG Yuqing. EFFECT OF FINISH COOLING TEMPERATURE ON MICROSTRUCTURE AND LOW TEMPERATURE TOUGHNESS OF Mn-SERIES ULTRA-LOW CARBON HIGH STRENGTH LOW ALLOYED STEEL. Acta Metall Sin, 2015, 51(1): 21-30.
Recently, the steel plates used in the ship, pipeline and bridge generally required not only high strength but also excellent low temperature toughness. As a competitive candidate, the ultra-low carbon high strength low alloyed (HSLA) steel has been developed widely. The low temperature toughness depends on the microstructure of the steels. Therefore, the relationship of low temperature toughness and microstructure should be studied in detail. In the present work, the steel plates with 25 mm thickness after hot rolling were immediately water quenched to 550, 450 and 350 ℃(finish cooling temperature), respectively, and subsequently air cooled to room temperature. The effect of finish cooling temperature on the microstructure and low temperature toughness of Mn-series ultra-low carbon HSLA steel was investigated by SEM, TEM and crystallographic analysis. The results show that the granular bainite, lath bainite and martensite were obtained with finish cooling temperatures decreasing. There are three blocks with different orientations in a single packet for lath bainite microstructure in the sample with finish cooling temperature of 450 ℃, leading to the refinement of effective grain size and large amount of high-angle grain boundaries. Electron backscattered diffraction analyses of the cleavage crack path show that the bainite block boundaries can strongly hinder fracture propagation, and thus the refinement of bainite blocks can improve the low temperature toughness of Mn-series ultra-low carbon HSLA steel. Finally, the yield strength of 775 MPa and ductile-brittle transition temperature of -55 ℃can be achieved when the finish cooling temperature is 450 ℃.
Fig.1 Ductile-brittle transition curves of the ULC-HSLA steel under different finish cooling temperatures
FCT ℃
Tensile strength MPa
Yield strength MPa
Yield ratio
Elongation %
550
972
745
0.766
21.5
450
990
775
0.782
17.5
350
996
783
0.786
17.0
Table 1 Tensile properties of the ultra-low carbon high strength low alloyed (ULC-HSLA) steel under different finish cooling temperatures
Fig.2 SEM (a, c, e) and TEM (b, d, f) images of the ULC-HSLA steel with the finish cooling temperatures of 550 ℃ (a, b), 450 ℃ (c, d) and 350 ℃ (e, f) (M/A—martensite/austenite island, LB—lath bainite, M—martensite)
Variant
Plane parallel
Direction parallel
Rotation from Variant 1
Angle / (o)
V1
(111)fcc∥(011)bcc
[101]fcc∥[111]bcc
-
-
V2
[101]fcc∥[111]bcc
[0.58 -0.58 0.58]
60.0
V3
[011]fcc∥[111]bcc
[0.0 -0.71 -0.71]
60.0
V4
[011]fcc∥[111]bcc
[0.0 0.71 0.71]
10.5
V5
[110]fcc∥[111]bcc
[0.58 0.71 0.71]
60
V6
[110]fcc∥[111]bcc
[0.0 -0.71 -0.71]
49.5
V7
(111)fcc∥(011)bcc
[101]fcc∥[111]bcc
[-0.58 -0.58 0.58]
49.5
V8
[101]fcc∥[111]bcc
[0.58 -0.58 0.58]
10.5
V9
[110]fcc∥[111]bcc
[-0.19 0.77 0.61]
50.5
V10
[110]fcc∥[111]bcc
[-0.49 -0.46 0.74]
50.5
V11
[011]fcc∥[111]bcc
[0.35 -0.93 -0.07]
14.9
V12
[011]fcc∥[111]bcc
[0.36 -0.71 0.60]
57.2
V13
(111)fcc∥(011)bcc
[011]fcc∥[111]bcc
[0.93 0.35 0.07]
14.9
V14
[011]fcc∥[111]bcc
[0.74 0.46 -0.49]
50.5
V15
[101]fcc∥[111]bcc
[-0.25 -0.63 -0.74]
57.2
V16
[101]fcc∥[111]bcc
[0.66 0.66 0.36]
20.6
V17
[110]fcc∥[111]bcc
[-0.66 0.36 -0.66]
51.7
V18
[110]fcc∥[111]bcc
[-0.3 -0.63 -0.72]
47.1
V19
(111)fcc∥(011)bcc
[110]fcc∥[111]bcc
[-0.61 0.19 -0.77]
50.5
V20
[110]fcc∥[111]bcc
[-0.36 -0.6 -0.71]
57.2
V21
[011]fcc∥[111]bcc
[0.96 0.0 -0.30]
20.6
V22
[011]fcc∥[111]bcc
[0.72 0.3 -0.63]
47.1
V23
[101]fcc∥[111]bcc
[0.74 -0.25 0.063]
57.2
V24
[101]fcc∥[111]bcc
[0.91 -0.41 0.0]
21.1
Table 2 Twenty-four variants in K-S relationship[15]
Fig.3 EBSD analysis of the sample with finish cooling temperature of 550 ℃
Fig.4 Boundary map of the sample with finish cooling temperature of 550 ℃(The white, yellow, blue and red lines indicate the prior austenite grain boundaries and the boundaries with misorientations of 2°~10°, 10°~15° and higher than 15°, respectively) (a), point-to-origin A misorientation of A-B line (b) and point-to-point misorientation of A-B line (c)
Fig.5 EBSD analysis of the sample with finish cooling temperature of 450 ℃
Fig.6 Boundary map of the sample with finish cooling temperature of 450 ℃(The white, yellow, blue and red lines indicate the prior austenite grain boundaries and the boundaries with misorientations of 2°~10°, 10°~15° and higher than 15°, respectively) (a), point-to-origin A misorientation of A-B line (b) and point-to-point misorientation of A-B line (c)
Fig.7 SEM image and EBSD analysis of the sample with finish cooling temperature of 450 ℃
Fig.8 EBSD analysis of the sample with finish cooling temperature of 350 ℃
Fig.9 SEM images of cross-sectional area beneath cleavage fracture surface of Charpy impact specimen fractured at -80 ℃for the samples with finish cooling temperatures of 550 ℃(a) and 350 ℃(b) (AGB—prior austenite grain boundary)
Fig.10 SEM image (a) and corresponding orientation image obtained by EBSD analysis (b) of cross-sectional area beneath cleavage fracture surface of Charpy impact specimen fractured at -80 ℃ for the samples with finish cooling temperature of 450 ℃(The arrows indicate the propagation of cleavage cracks, the misorientations between adjacent zones are 53.03°, 50.06°, 58.30°, 57.8°, 56.02°, 60.00° and 47.40°, respectively)
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