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Acta Metall Sin  2020, Vol. 56 Issue (4): 653-660    DOI: 10.11900/0412.1961.2019.00398
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Quantitative Crystallographic Characterization of Boundaries in Ferrite-Bainite/Martensite Dual-Phase Steels
LI Xiucheng,SUN Mingyu,ZHAO Jingxiao,WANG Xuelin,SHANG Chengjia()
Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China
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Abstract  

In this study, two dual-phase steels with different ferrite-bainite/martensite ratios were obtained by rolling in two-phase region and setting the relaxation time after rolling. The tested steel with smaller ferrite content obtained higher yield strength and tensile strength, greater total elongation and lower ductile-brittle transition temperature; while the steel with higher ferrite content obtained higher uniform elongation and lower yield strength ratio. The EBSD characterization of the two steels shows that for the ferrite-ferrite boundaries and ferrite-bainite/martensite boundaries, if the interface has a large overall misorientation angle, it usually has a large cleavage plane misorientation angle and large slip plane misorientation angle; but for the variant-variant boundaries within bainite or martensite, if the interface has a large overall misorientation angle, it usually has a large cleavage plane misorientation angle, but not necessarily has a large slip plane misorientation angle, and this phenomenon is more significant in martensite microstructure. The ductility of dual-phase steel is not only affected by the proportion of the two phases, but also influenced by the grain refinement of the two phases. Therefore, in order to improve the comprehensive mechanical properties of the dual phase steel, it is necessary to refine the dual phase microstructure from the view of effective slip unit and the effective cleavage unit.

Key words:  dual-phase steel      microstructure      grain boundary      misorientation      grain refinement     
Received:  22 November 2019     
ZTFLH:  TG142  
Fund: National Key Research and Development Program of China(2017YFB0304900)
Corresponding Authors:  Chengjia SHANG     E-mail:  cjshang@ustb.edu.cn

Cite this article: 

LI Xiucheng,SUN Mingyu,ZHAO Jingxiao,WANG Xuelin,SHANG Chengjia. Quantitative Crystallographic Characterization of Boundaries in Ferrite-Bainite/Martensite Dual-Phase Steels. Acta Metall Sin, 2020, 56(4): 653-660.

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https://www.ams.org.cn/EN/10.11900/0412.1961.2019.00398     OR     https://www.ams.org.cn/EN/Y2020/V56/I4/653

PassDQ810AC730

Thickness

mm

Temperature

Rolling reduction ratio / %

Thickness

mm

Temperature

Rolling reduction ratio / %
0801200-801200-
168110015.068110015.0
255105019.755105019.7
344100020.044100020.0
43395025.03495022.7
52783018.22883021.4
62382014.82481014.3
72081013.0227408.3
8---207309.1
Table 1  Rolling schedule of the two steels samples
Fig.1  OM images of microstructures of DQ810 (a) and AC730 (b) steels
Fig.2  Low magnification EBSD maps and grain boundaries with overall misorientation angle above 5° (red lines) of DQ810 (a) and AC730 (b) steelsColor online

Steel

Yield strength

MPa

Tensile strength

MPa

Yield ratio

Uniform elongation

%

Total elongation

%

DQ8105507990.6910.125.6
AC7304577530.6111.922.8
Table 2  Tensile properties of the two steels
Steel-40 ℃-60 ℃-80 ℃
DQ810147, 149, 181 (average: 159)123, 139, 161 (141)28, 43, 14 (28)
AC73050, 44, 38 (44)22, 39, 15 (25)17, 16, 11 (15)
Table 3  Charpy impact toughness of the two steels at a series of low temperatures
Fig.3  EBSD map of DQ810 sample and grain boundaries above 5° (red lines) (a) and misorientation angle of every boundaries along the line between A and B in Fig.3a (b) (F—ferrite, B—bainite, V—variant within bainite or martensite)Color online
Fig.4  EBSD map of AC730 sample and grain boundaries above 5° (red lines) (a) and misorientation angle of every boundaries along the line between A and B in Fig.4a (b)Color online
Fig.5  Boundaries density (histogram) and distribution (line) with overall misorientation angle in DQ810 (a) and AC730 (b) steelsColor online
Fig.6  Boundaries density (histogram) and distribution (line) with cleavage plane misorientation angle in DQ810 (a) and AC730 (b) steelsColor online
Fig.7  Boundaries density (histogram) and distribution (line) with slip plane misorientation angle in DQ810 (a) and AC730 (b) steelsColor online
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