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Acta Metall Sin  2017, Vol. 53 Issue (8): 937-946    DOI: 10.11900/0412.1961.2017.00038
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Strengthening Mechanism of 600 MPa Grade Nb-Ti Microalloyed High Formability Crossbeam Steel
Yajun HUI1,2(), Hui PAN1, Kun LIU1, Wenyuan LI1, Yang YU1, Bin CHEN1, Yang CUI1
1 Sheet Metal Research Institute, Shougang Research Institute of Technology, Beijing 100043, China
2 Beijing Key Laboratory of Green Recyclable Process for Iron & Steel Production, Shougang Group, Beijing 100043, China
Cite this article: 

Yajun HUI, Hui PAN, Kun LIU, Wenyuan LI, Yang YU, Bin CHEN, Yang CUI. Strengthening Mechanism of 600 MPa Grade Nb-Ti Microalloyed High Formability Crossbeam Steel. Acta Metall Sin, 2017, 53(8): 937-946.

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Abstract  

Automobile beam steel with high strength is the development trend of the automotive industry, which will help to conserve resources and protect the environment. The crossbeam as an important part of the frame, its strength also affects the overall strength of the frame, which will affect the safety performance of vehicles. At present, the crossbeam steel of heavy-duty vehicles is mainly Q235B and Q345C. About 3% of the crossbeams occur cracking problems when heavy-duty vehicles drive about 10000 km or about half a year or so. The strength increasing will cause the cracking in the forming process. Therefore, it is of great significance to develop a high strength crossbeam steel with high fatigue and high formability. In this work, the microstructure, properties and strengthening mechanism of 600 MPa grade Nb-Ti microalloyed high formability crossbeam steel were investigated by OM, SEM and TEM. The results show that the finish rolling temperature has a considerable influence on the microstructure and mechanical strength, with the decreasing of finish rolling temperature, the ferrite grain size and the size of precipitates decreases gradually; dislocation density, the number of precipitates and the ratio of Nb/Ti increase gradually; both the yield strength and tensile strength increase monotonously, while the elongation has an optimum temperature. The optimal mechanical properties were obtained when the finish rolling temperature is 840 ℃, and the yield strength, tensile strength, elongation and impact energy at -60 ℃ reached 541 MPa, 615 MPa, 31.0% and 117 J, respectively. The NrT and PTT curves of (Nb, Ti)C precipitated in the austenite showed that the nucleation rate of uniform nucleation and dislocation linear nucleation increases and the nucleation time is shortened with the decrease of temperature in the experimental temperature range, which is consistent with the observation result that the size of precipitates decreases with the reduction of the finish rolling temperature, while the number of precipitates increases. The grain refinement strengthening and dislocation strengthening are the main strengthening modes of the steel, the grain refinement strengthening accounts for 46%~48% of the yield strength, the dislocation strengthening accounts for 18%~25%, while the precipitation strengthening only contributes 2% of the yield strength.

Key words:  600 MPa grade      Nb-Ti microalloyed      high formability      crossbeam steel      strengthening mechanism      finish rolling temperature     
Received:  13 February 2017     
ZTFLH:  TG142.1  

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00038     OR     https://www.ams.org.cn/EN/Y2017/V53/I8/937

Finish rolling Yield Tensile Elongation Yield ratio 180° d=a
temperature strength strength % cold bending
MPa MPa
920 464 541 29.5 0.858 OK
880 510 583 31.5 0.875 OK
840 541 615 31.0 0.878 OK
800 555 622 27.5 0.892 OK
Table 1  Mechanical properties of the experimental steel
Fig.1  Charpy impact energy of the experimental steel
Fig.2  OM (a, c, e, g) and SEM (b, d, f, h) images of the experimental steels with finish rolling temperatures of 920 ℃ (a, b), 880 ℃ (c, d), 840 ℃ (e, f) and 800 ℃ (g, h)
Fig.3  TEM images (a, c, e, g) and EDS analyses (b, d, f, h) of precipitates in the experimental steels with finish rolling temperatures of 920 ℃ (a, b), 880 ℃ (c, d), 840 ℃ (e, f) and 800 ℃ (g, h)
Fig.4  TEM images of the distribution of precipitates in the experimental steels with finish rolling temperatures of 920 ℃ (a) and 840 ℃ (b)
Fig.5  STEM images of dislocation in the experimental steels with finish rolling temperatures of 920 ℃ (a) and 840 ℃ (b)
Fig.6  NrT (a) and PTT (b) curves of (Nb, Ti)C precipitate in austenite of the experimental steel(I—nucleation rate, K—constant, I/K—relative nucleation rate, ta—nucleation start time when nucleation rate attenuation rapidly, t0a—a temperature-independent constant when nucleation rate attenuation rapidly, ta/t0a—relative nucleation start time when nucleation rate attenuation rapidly)
Finish Rolling σ0 ΔσS ΔσG ΔσDis ΔσOrowan σy Measured σy
temperature / ℃ MPa MPa MPa MPa MPa MPa MPa
920 48 98 220 85 10 461 464
840 48 98 255 136 14 551 541
Table 2  Yield strength and calculated values of its component of the steel with different finish rolling temperatures
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