Microstructure Characteristic and Strengthening Mechanism of Ti-Microalloyed Automobile Beam Steel Produced by Compact Strip Production

[10] 2/1− = dkygσ (2) where ky is the grain boundary unlocking term, for high angle grain boundaries, taken as 15.1-18.1, in this paper ky =15.1Nmm-3/2, d is the ferrite grain size, �m. The average ferrite grain size of sample E is about 5.0�m measured by mean linear intercept (a) (b) method, so the contribution of refinement strengthening to the yield strength of sample E is 213.5MPa. It can be seen that this strengthening mode plays a major role in the final yield strength, and this is the main reason for automobile beam steel with good mechanical and toughness properties. Solid solution strengthening consists of interstitial solution strengthening and substitutional solution strengthening. According to lots of research work, solid solution strengthening in common dilute solution can be described as: ][9.2][30][113 ][5.80][11][38][470][83][37][3750][4570 VCrSn Ti MoCuPSiMnNCss +++ +++++++=σ (3) Where [x] is the mass fraction of x solute. Based on the above equation, consulting the chemical composition of sample E and subtracting the precipitated alloy (Table 4), the calculated value of solid solution strengthening is 96.6MPa. The yield strength of steel with contents of carbon from 0.05-0.25% is quantitatively depicted by the following equation[11]: dp free s d N Si Mn MPa σσ σ ++ + ++ += − 2/1 1.15)(%2900)(%83)(%2788)( (4) Where %Nfree is content of free nitrogen in steel, d is ferrite grain size; σp is the precipitation strengthening, σd is the dislocation strengthening. Based on the equation 4, we can estimate the contribution of the precipitation strengthening and dislocation strengthening to the yield strength of automobile beam steel. The calculated result is about 116.8MPa. It is indicated that precipitation and dislocation strengthening components have a significant contribution to the yield strength of automobile beam steel produced by CSP. Summary The microstructure of hot strip produced by CSP consists of polygonal ferrite and a small amount of pearlite. Coiling temperature and cooling rate greatly influence the microstructure of hot strips after controlled rolling. At low cooling rates, the microstructures of steels consist of banded ferrite and pearlite along the rolling direction but higher cooling rates suppress its formation. Refinement strengthening is the predominant strengthening mode among all the strengthening modes, solid solution strengthening, precipitation strengthening and dislocation strengthening are second to it. Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China (No. 50334010). Refenerces

Google Scholar

[1] G. Flemming, K.E. Hensger: MPT Inter., vol.1(2000), p.54

Google Scholar

[2] Yonglin Kang, Hao Yu, Jie Fu, et al: Mater. Sci. Eng., vol.A351 (2003), p.265

Google Scholar

[3] Hao Yu, Yonglin Kang, Kelu Wang, et al: Mater. Sci. Eng., Vol.A363 (2003), p.86

Google Scholar

[4] B. Gardiola, M. Humbert, C. Esling, et al: Mater. Sci. Eng., vol.A303 (2001), p.60

Google Scholar

[5] P.J. Hurley, P.D. Hodgson: Mater. Sci. Eng., vol.A 302 (2001), p.206

Google Scholar

[6] L. Storojeva, R. Kaspar, D. Ponge, ISIJ Int., vol.44 (2004), p.1211

Google Scholar

[7] M. Thompson, M. Ferry, P. A. Manohar. ISIJ Int., vol.41 (2001), p.891 [8] Hao Yu, Yonglin Kang, Xueying Xiong, et al: J.Univ. Sci. Technol. Beijing, vol.11 (2004), p.425

Google Scholar

[9] N. Gao, T.N. Baker: ISIJ Int., vol.38 (2001), p.744

Google Scholar

[10] J. Majta, J.G. Lenard, M. Pietrzyk: Mater. Sci. Eng., vol.A208 (1996), p.249

Google Scholar

[11] T. Gladman, D. Dulieu, I.D Mclvor: Microalloying, (1977), p.32

Google Scholar