Improving Tensile Properties of Room-Temperature Quenching and Partitioning Steel by Dislocation Engineering
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The strength of advanced high strength steel (AHSS) plays a key role in the weight reduction of structural components of automobiles.[1] However, improving the strength of AHSS frequently results in a reduction in ductility.[2] Hence, pathways to increasing dislocation density can significantly increase material strength but often with a concomitant loss in ductility.[3,4] Nevertheless, it has been demonstrated recently that high dislocation density can not only increase strength but also improve ductility in steels.[5,6] High dislocation density is mainly responsible for improved yield strength through dislocation forest hardening, while improved ductility is achieved by the glide of intensive mobile dislocations and well-controlled martensitic transformation, both of which are governed by the high dislocation density resulting from warm rolling and displacive shear transformation.[5] Consequently, a new alloy design
B.B. HE, M. WANG, and M.X. HUANG are with the Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, China and also with the Shenzhen Institute of Research and Innovation, The University of Hong Kong, Shenzhen 518000, China. Contact e-mail: [email protected] Manuscript submitted January 16, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
concept which is known as ‘‘dislocation engineering’’ is proposed.[7] The dislocations generated during the warm rolling process can be taken over by the martensite either transformed during the cold rolling process[5,8] or the quenching process.[9,10] The martensitic transformation triggered by the quenching process is a critical step in developing the quenching and partitioning (Q&P) steel.[11–13] Therefore, it has been proposed that the dislocation engineering concept can be used to improve the tensile properties of Q&P steel.[7] The quenching temperature is very important for the mechanical properties of Q&P steel as it mainly determines the volume fraction of constituting phases.[14] Generally, the quenching temperature is higher than room temperature,[15–17] making the thermal processing of Q&P steel relatively complicated. Recently, it has been found that this quenching temperature can be decreased down to room temperature by tuning the austenite stabilizing elements, resulting in a new room-temperature quenching and partitioning (RT Q&P) steel grade.[18,19] For the first time, the dislocation engineering concept is applied to the RT Q&P process in present work. It is found that the dislocations generated through a warm rolling process are taken over by the martensite matrix and retained austenite after subsequent water quenching down to room temperature. These dislocations can refine the martensite block size by resisting the glissile transformation interface.[20,21] Similarly, the austenite volume fraction is increased after the warm rolling process owing to the mechanical stabilization.[20] It is expected that such dislocation engineering in both the martensite and austenite phases will allow the production of strong and ductile steels. A
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