The Effect of Nano-precipitates on Strength in a Micro-alloyed Ferritic Steel
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The Effect of ano-precipitates on Strength in a Micro-alloyed Ferritic Steel H.A. Askari1 , Y.F. Shen2, C.M. Wang3, X. Sun3, H.M. Zbib1 1
Mechanical and Materials Engineering, Washington State University, Pullman, WA. Key Laboratory for Anisotropy and Texture of Materials (Ministry of Education), Northeastern University, Shenyang 110004, P.R. China 3 Pacific Northwest National Laboratory, PO Box 999, Richland, WA 99352, USA 2
ABSTRACT A high strength ferritic steel with finely dispersive precipitates was investigated to reveal the fundamental strengthening mechanisms in this alloy. Using energy dispersive X-ray spectroscopy (EDXS) and transmission electron microscope (TEM), fine carbides with an average diameter of 10 nm were observed in the ferrite matrix of the 0.08%Ti steel, and some cubic M23C6 precipitates were also observed at the grain boundaries and the interior of grains. The dual precipitate structure of finely dispersive TiC precipitates in the matrix and coarse M23C6 at grain boundaries provides combined matrix and grain boundary strengthening. The resulting yield stress is two or three times higher than that of conventional Ti-bearing high strength hot-rolled sheet steels. The effect of the particle size, particle distribution and intrinsic particle strength have been investigated through dislocation dynamics (DD) simulations and the relationship for resolved shear stress for single crystal under this condition has been presented using simulation data. The results show that the finely dispersive precipitates can strengthen the material by pinning the dislocations up to a certain shear stress and retarding the recovery as well as annihilation of dislocations. The DD results also show that strengthening is not only a function of the density of the nano-scale precipitates but also of their size. This size effect is explained using a mechanistic model developed based on dislocation-particle interaction. I TRODUCTIO
In past decades, the ongoing research of the steel industry mainly focuses on high strength steels with excellent formability. The good combination of strength and ductility is particularly attractive for automotive applications such as structural reinforcements and energy absorption parts. Among various measures taken, the reduction of the vehicle weight is proved to be the most effective, which can be achieved by developing and employing a new generation of costeffective lightweight steels called Advanced High Strength Steels (AHSS). It is well established that AHSS are strengthened by a combination of factors: grain refinement, solid solution strengthening and precipitation hardening. AHSS usually possess yield strengths of about 400-500 MPa, and the contribution of precipitation hardening to these values was considered to be minor, since many of the alloying elements were added to AHSS in the past basically for strengthening through grain refinement. Recently, yield strengths of up to 780MPa have been achieved in Ti and Mo bearing AHSS sheet steels by producing microstructures that consist
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