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Acta Metall Sin  2016, Vol. 52 Issue (5): 529-537    DOI: 10.11900/0412.1961.2015.00411
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EFFECT OF COILING TEMPERATURE ON MICRO-STRUCTURE AND MECHANICAL PROPERTIES OF Ti-V-Mo COMPLEX MICROALLOYED ULTRA-HIGH STRENGTH STEEL
Ke ZHANG1,2,Qilong YONG2(),Xinjun SUN2,Zhaodong LI2,Peilin ZHAO3
1 School of Materials Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China
2 Department of Structural Steels, Central Iron and Steel Research Institute, Beijing 100081, China
3 R&D Center, Laiwu Iron and Steel Group Co. Ltd., Laiwu 271104, China
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Abstract  

Among various hardening factors of steels, precipitation hardening has the least embrittlement vector value except grain refinement hardening. Giving full play to the precipitation hardening of microalloyed carbonitrides is an important aspect in the development of microalloyed high strength steels. Recently, the research on behaviors of precipitation and development of microalloyed high strength steels is mainly focused on these relatively simple microalloyed steels including single V, single Ti, Ti-V and Ti-Mo microalloyed steels, while paid less attention on complex microalloyed steels such as Ti-V-Mo steels. Therefore, it is expected to provide a theoretical basis and a practical significance for the development of Ti-V-Mo microalloyed high strength steel. Various hardening increments at different coiling temperatures were calculated. Meanwhile, the effect of coiling temperatures on yield strength and the influence of MC particles on uniform elongation were discussed by means of OM, EBSD, TEM, XRD and physical-chemical phase analysis. The results show that Ti-V-Mo steel has the best mechanical properties with ultimate tensile strength of 1134 MPa, yield strength of 1080 MPa, elongation of 13.2% and uniform elongation of 6.8% at coiling temperature of 600 ℃. The precipitation hardening increment was high to about 444~487 MPa due to the mass fraction of about 72.6% of total precipitates with a size of ?10 nm. In addition, precipitation hardening and grain refinement hardening are the main mechanisms to improve the strength of Ti-V-Mo steel, while the variation in precipitation hardening increment causes a significent difference in yield strength. With the coiling temperature increases from 500 ℃ to 600 ℃, the ultimate tensile strength and yield strength increase continuously, but the uniform elongation increases slowly instead of decreasing, which is mainly attributed to an increase of precipitation hardening increment.

Key words:  coiling temperature      precipitation hardening      yield strength      uniform elongation      Ti-V-Mo     
Received:  23 July 2015     
Fund: Supported by National Basic Research Program of China (No.2015CB654803), National Natural Science Foundation of China (No.51201036) and National Science and Technology Pillar Program (No.2013BAE07B05)

Cite this article: 

Ke ZHANG,Qilong YONG,Xinjun SUN,Zhaodong LI,Peilin ZHAO. EFFECT OF COILING TEMPERATURE ON MICRO-STRUCTURE AND MECHANICAL PROPERTIES OF Ti-V-Mo COMPLEX MICROALLOYED ULTRA-HIGH STRENGTH STEEL. Acta Metall Sin, 2016, 52(5): 529-537.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00411     OR     https://www.ams.org.cn/EN/Y2016/V52/I5/529

Fig.1  Schematic of thermomechanical controlled process of Ti-V-Mo steel
Fig.2  OM images of Ti-V-Mo steel at coiling temperatures of 500 ℃ (a), 550 ℃ (b), 600 ℃ (c) and 650 ℃ (d) (Inset in Fig.2a shows the enlarged image)
Fig.3  EBSD images of Ti-V-Mo steel at coiling temperatures of 500 ℃ (a), 550 ℃ (b), 600 ℃ (c) and 650 ℃ (d) (Red line—low angle grain boundary (2°≤ θ <15°), black line—high angle grain boundary (θ ≥15°) )
Fig.4  Mechanical properties of Ti-V-Mo steel at different coiling temperatures (σb—ultimate tensile strength, σy—0.2% yield strength, δ—total elongation, δgt—uniform eonglation)
Coiling MC M3C
temperature Ti* Mo V C* Fe Mn Mo V C*
500 ℃ 0.074 0.062 0.059 0.040 0.235 0.767 0.031 0.010 0.008 0.058 0.874
550 ℃ 0.095 0.116 0.105 0.052 0.322 0.516 0.029 0.012 0.015 0.041 0.613
600 ℃ 0.139 0.240 0.254 0.125 0.757 0.167 0.022 0.018 0.017 0.016 0.240
650 ℃ 0.141 0.238 0.257 0.126 0.761 0.122 0.018 0.020 0.019 0.012 0.191
Table 1  Quantitative analysis results of precipitates MC and M3C at different coiling temperatures (mass fraction / %)
Fig.5  TEM image of nano-sized precipitates (a) and size distribution of (Ti, V, Mo)C particles (b) at coiling temperature of 600 ℃
Particle size / nm Mass fraction / % Volume fraction / % σp / MPa
1~5 49.4 0.4727 445
5~10 23.2 0.2220 164
10~18 24.7 0.2363 110
18~36 0.4 0.0038 9
Table 2  Calculations results of precipitation hardening increments (σp) of different size intervals coiled at 600 ℃
Coiling σo σs σg σp σd σexp σp /σexp
temperature
500 ℃ 48 165 370 214 63 860 0.249
550 ℃ 48 162 368 291 63 932 0.312
600 ℃ 48 145 380 444 63 1080 0.411
650 ℃ 48 158 370 326 63 965 0.338
Table 3  Various strengthening increments of different coiling temperatures
Steel (mass fraction / %) Processing σp / MPa σg / MPa σb / MPa Ref.
0.04C-0.092Ti-0.19Mo Laboratory FRT at 900 ℃ 300 312 820 [2]
and coiled at 620 ℃
0.075C-0.17Ti-0.275Mo Laboratory FRT at 880 ℃ 276 318 951 [9]
and coiled at 620 ℃
0.059C-0.23Ti-0.19Mo Laboratory FRT at 900 ℃ and ?200 365 769 [11]
coiled at 620 ℃
0.096C-0.25Ti-0.45Mo- Laboratory FRT at (900 ±10) ℃ 330~430 420~450 1020~1170 [12]
0.031Nb-0.074V and coiled at 600 ℃
0.09C-0.093Ti-0.26Mo-0.14V Laboratory FRT at 780 ℃ and 310 361 955 [22]
coiled at 600 ℃
0.10C-0.10Ti-0.12Mo Laboratory FRT at 850~ 160 285 627 [31]
930 ℃ and coiled at 620 ℃
0.16C-0.20Ti-0.44Mo-0.41V Laboratory FRT at 870 ℃ and 444~487 380 1134 This
coiled at 600 ℃ work
Table 4  Strengthening increments of different Ti-Mo composition steels
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