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Acta Metall Sin  2016, Vol. 52 Issue (4): 410-418    DOI: 10.11900/0412.1961.2015.00482
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PRECIPITATION BEHAVIOR OF NANOMETER-SIZED CARBIDES IN Nb-Mo MICROALLOYED HIGH STRENGH STEEL AND ITS STRENGTHENING MECHANISM
Zhengyan ZHANG1,2,Xinjun SUN1(),Qilong YONG1,2,Zhaodong LI1,Zhenqiang WANG3,Guodong WANG2
1 Department of Structural Steels, Central Iron and Steel Research Institute, Beijing 100081, China
2 State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110819, China
3 College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China
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Abstract  

Recently, increasing attention has been focused on the high strength low alloy (HSLA) steels mircoalloyed with multiple miroalloying elements, such as Nb-Ti, Nb-V and Ti-Mo, which can form synthetic carbide in steel, such as (Nb, Ti)C, (Nb, V)C and (Ti, Mo)C. Compared with the simplex carbide, such as NbC, TiC, those synthetic carbides with nanometer size exhibiting a superior thermal stability to exert their powerful influence mainly through their precipitation hardening in ferrite. It is reported that the precipitation hardening of approximate 300 MPa which can be obtained in Ti-Mo-bearing steel was developed by JFE steel, attributing to the synthetic (Ti, Mo)C particle precipitated in ferrite. However, as common microalloying elements, Nb and Mo are added synchronously in steel. The strengthening mechanism of Nb-Mo mircoalloyed as-rolled steel and the role of the carbide precipitated in Nb-Mo mircoalloyed as-rolled steel are rarely reported. Therefore, in the present study, the strengthening mechanism, microstructure and the precipitate characteristics of Nb and Nb-Mo microalloyed steels produced by thermo mechanical control process (TMCP) were comparatively investigated by means of SEM, EBSD, HRTEM and physical and chemical phase analysis, in order to systematically study the synergistic effect of Nb-Mo addition on the strength of as-rolled steel. The results shows that the microstructure is finer and the density of low-angle grain boundaries is higher in Nb-Mo microalloyed steel compared with that of in the Nb microalloyed steel. What's more, the Mo addition could increase the precipitation ratio of Nb, and the amount of the MC-type carbide with nanometer size in Nb-Mo microalloyed steel is evidently larger than that of in Nb microalloyed steel. Those MC-type carbide were identified as synthetic carbide (Nb, Mo)C, exhibiting low coarsening rate than that of NbC precipitated in Nb microalloyed steel, which thus contributed to a higher precipitation hardening. This is main reason of the difference in strength between Nb and Nb-Mo microalloyed steel.

Key words:  Nb-Mo microalloyed      strength mechanism      nanometer-sized carbide      precipitation     
Received:  15 September 2015     
Fund: Supported by Supported by National Basic Research Program of China (No.2015CB654803) and High Technology Research and Development Program of China (No.2015AA034302)

Cite this article: 

Zhengyan ZHANG,Xinjun SUN,Qilong YONG,Zhaodong LI,Zhenqiang WANG,Guodong WANG. PRECIPITATION BEHAVIOR OF NANOMETER-SIZED CARBIDES IN Nb-Mo MICROALLOYED HIGH STRENGH STEEL AND ITS STRENGTHENING MECHANISM. Acta Metall Sin, 2016, 52(4): 410-418.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00482     OR     https://www.ams.org.cn/EN/Y2016/V52/I4/410

Steel C Mn P S Si Al Mo Ti Nb B Fe
Nb 0.036 1.35 0.0034 0.0057 0.024 0.012 - 0.010 0.1 0.0012 Bal.
Nb-Mo 0.042 1.38 0.0040 0.0060 0.016 0.014 0.19 0.015 0.1 0.0010 Bal.
Table 1  Chemical compositions of Nb and Nb-Mo microalloyed steels (mass fraction / %)
Fig.1  Tensile engineering stress-strain curves of Nb and Nb-Mo microalloyed steels at room temperature
Steel σb / MPa σ0.2 / MPa δ / % Yield ratio
Nb 679 630 23 0.92
Nb-Mo 739 699 21 0.94
Table 2  Mechanical properties of Nb and Nb-Mo microalloyed steels at room temperature
Fig.2  SEM images of Nb (a) and Nb-Mo (b) microalloyed steels (RD—rolling direction, ND—normal direction)
Fig.3  EBSD grain boundary distribution maps of Nb (a) and Nb-Mo (b) microalloyed steels (Where black and red lines represent the high misorientation angle boundaries (≥15°) and low misorientation angle boundaries (2°~15°), respectively) and the total grain boundary density of Nb and Nb-Mo microalloyed steels vs the misoritentation of ferrite grain ranged 0°~60° (c)
Fig.4  HRTEM image of a MC-type particle precipitated in austenite of Nb-Mo mircoalloyed steel (a) and its fast Fourier transformed (FFT) result along the [111]MC and [001]α-Fe zone axis (b)
Fig.5  TEM images of nanometer sized precipitates (a, b) and corresponding EDS (c, d) of Nb (a, c) and Nb-Mo (b, d) microalloyed steels
Fig.6  XRD spectra of precipitates in Nb and Nb-Mo microalloyed steels
Steel N3C (N=Fe, Mn, Mo) MC (M=Nb, Ti, Mo)
Fe Mn Mo C* Nb Ti Mo C*
Nb 0.177 0.032 - 0.015 0.076 0.0095 - 0.0098
Nb-Mo 0.168 0.022 0.014 0.014 0.078 0.0150 0.028 0.0133
Table 3  Element contents in precipitates of Nb and Nb-Mo microalloyed steels (mass fraction / %)
Fig.7  Size distribution of MC-type precipitate in Nb and Nb-Mo microalloyed steels
d / nm fm / % fV / % σp / MPa
Nb Nb-Mo Nb Nb-Mo Nb Nb-Mo
1~5 0.005246 0.04236 0.005234 0.04230 43.0 122.1
5~10 0.006192 0.00996 0.006178 0.00994 27.3 34.6
10~18 0.007826 0.01560 0.007808 0.01560 20.0 28.2
18~36 0.014448 0.00492 0.014415 0.00491 16.7 9.8
36~60 0.011008 0.00948 0.010983 0.00946 9.3 8.7
Table 4  Volume fractions of precipitates with different sizes and the corresponding precipitation strengthening increments in Nb and Nb-Mo microalloyed steels
Fig.8  Analysis and comparison of the strengthening mechanism for Nb and Nb-Mo microalloyed steels
Fig.9  Ripening rate of precipitates vs temperature in ferrite of Nb and Nb-Mo microalloyed steels
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