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Acta Metall Sin  2018, Vol. 54 Issue (1): 31-38    DOI: 10.11900/0412.1961.2017.00202
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Effect of Cooling Rate on Microstructure Evolution and Mechanical Properties of Ti-V-Mo Complex Microalloyed Steel
Ke ZHANG1,2(), Zhaodong LI3, Fengli SUI1,2, Zhenghai ZHU1,2, Xiaofeng ZHANG1,2, Xinjun SUN3, Zhenyi HUANG1,2, Qilong YONG3
1 Key Laboratory of Metallurgical Emission Reduction & Resources Recycling (Anhui University of Technology), Ministry of Education, Maanshan 243032, China;
2 School of Metallurgical Engineering, Anhui University of Technology, Maanshan 243032, China
3 Institute of Structural Steels, Central Iron and Steel Research Institute, Beijing 100081, China
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Abstract  

Nanoscale co-precipitation strengthening in steels has attracted increasing attention in recent years and has become a new cornerstone for the development of advanced high performance steels with superior combination of strength and ductility. Rolling process, finishing temperature, cooling rate and coiling temperature are the main factors which affect the mechanical properties of microalloyed steels by changing the volume fraction and particle size of precipitates. Nevertheless, the influence of cooling rate on microstructure evolution, precipitates and mechanical properties of complex microalloyed ferritic Ti-V-Mo steel has been rarely reported. In this work, the precipitation law of (Ti, V, Mo)C carbides at different cooling rates and its effect on microstructue evolution and mechanical properties of Ti-V-Mo complex miroalloyed steel were studied by OM, EBSD, HRTEM and Vickers-hardness test. The results indicated that the hardness first increased quickly and then increased slowly as the cooling rate increased (lower than 20 ℃/s); the mean size of (Ti, V, Mo)C particles decreased from 13.2 nm to 6.9 nm and the average size of ferrite grain reduced from 5.06 μm to 2.97 μm; the hardness of Ti-V-Mo steel was improved by the means of grain refinement hardening and precipitation hardening. However, when the cooling rate increased from 20 ℃/s to 30 ℃/s, its effects on grain refinement hardening and precipitation hardening has become saturated, so the hardness was kept flat and achieved a maximum vlaue of 410 HV, and the yield strength reached as high as 1090 MPa. The hardness y of Ti-V-Mo microalloyed steel and cooling rates x accord with a exponential decay relationship: y=-229exp(-x/5)+412.

Key words:  cooling rate      Ti-V-Mo      hardness      precipitate      ferrite     
Received:  24 May 2017     
ZTFLH:  TG142.1  
Fund: Supported by National Natural Science Foundation of China (Nos.51704008 and 51674004), National Key Research and Development Program of China (Nos.2017YFB0305100 and 2017YFB0304700), National Basic Research Program of China (No.2015CB654803), Science and Technology Foundation of China Iron & Steel Research Institute Group (No.15G60530A) and the Youth Scientific Research Foundation of Anhui University of Technology (No.QZ201603)

Cite this article: 

Ke ZHANG, Zhaodong LI, Fengli SUI, Zhenghai ZHU, Xiaofeng ZHANG, Xinjun SUN, Zhenyi HUANG, Qilong YONG. Effect of Cooling Rate on Microstructure Evolution and Mechanical Properties of Ti-V-Mo Complex Microalloyed Steel. Acta Metall Sin, 2018, 54(1): 31-38.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00202     OR     https://www.ams.org.cn/EN/Y2018/V54/I1/31

Fig.1  Schematic of thermo-mechanical controlled processing (TMCP) of Ti-V-Mo steel
Fig.2  OM images of Ti-V-Mo steel at the cooling rates of 1 ℃/s (a), 5 ℃/s (b), 10 ℃/s (c), 15 ℃/s (d), 20 ℃/s (e) and 30 ℃/s (f)
Fig.3  EBSD images of Ti-V-Mo steels at the cooling rate of 1 ℃/s (a), 5 ℃/s (b), 15 ℃/s (c) and 25 ℃/s (d) (Black and blue lines indicate the high misorientation angle boundaries (θ≥15°) and low misorientation angle boundaries (2°≤θ<15°), respectively)
Fig.4  Misorientation angle boundaries distribution of Ti-V-Mo steels at different cooling rates
Fig.5  Precipitates of Ti-V-Mo steels at the cooling rates of 1 ℃/s (a), 5 ℃/s (b), 15 ℃/s (c) and 25 ℃/s (d), and morphology (e) and corresponding EDS (f) of the particle in Fig.5d
Fig.6  Size distribution of precipitates of Ti-V-Mo steels at different cooling rates
Fig.7  Hardness of of Ti-V-Mo steel at different cooling rates
Fig.8  Variation of yield strength as a function of cooling rates of Ti-V-Mo steel
Cooling rate
℃s-1
Hardness
HV
Mean size of
precipitate / nm
Average grain
size / μm
1 225 13.2 5.06
5 330 11.3 4.54
15 394 6.9 2.97
25 410 6.4 2.63
Table 1  Microstructure parameters of Ti-V-Mo steel at different cooling rates
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