Benefits of Intercritical Annealing in Quenching and Partitioning Steel
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The quenching and partitioning (Q&P) steel was first proposed by Speer and co-workers[1] and has been considered to be a third-generation advanced high strength steel (AHSS) grade for lightweight automotive applications. The original Q&P concept[1] involves a full austenization annealing at a temperature above Ac3 followed by a fast quenching to a temperature between the martensite start (Ms) and finish (Mf) temperatures. The quenching step forms a microstructure consisting of retained austenite grains embedded in the martensite matrix. Finally, a partitioning step at or above the Ms temperature is employed to allow carbon partitioning from the martensite to the retained austenite. The martensite matrix in the Q&P steel can offer high strength, while the retained austenite provides the transformation-induced plasticity (TRIP) effect for improving ductility.[2] Most existing studies on the Q&P steel focus on tailoring the retained austenite in order to improve the mechanical properties.[3] These studies consider that the Q&P steel contains only a martensite matrix with retained austenite. Nevertheless, a high cooling rate from the austenization temperature
X. WANG and R.D. LIU are with the Iron and Steel Research Institute, Ansteel Group, Anshan, China. L. LIU and M.X. HUANG are with the Department of Mechanical Engineering, the University of Hong Kong, Hong Kong, China. Contact e-mail: [email protected] Manuscript submitted September 16, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
to the quenching temperature is required to achieve an ideal microstructure consisting of only the martensite matrix and the retained austenite. Such a high cooling rate can be achieved for small samples fabricated in laboratories. However, most existing industrial production lines (continuous annealing lines) in the steel industry cannot realize such a high cooling rate. It has been shown that the Q&P steel produced in the current industrial production line was cooled slowly from the fully austenitization region to the intercritical region. Such slow cooling results in the formation of proeutectoid ferrite, as well as a nonuniform carbon distribution in the retained austenite, leading to the unsatisfactory mechanical properties.[4] Therefore, it has been suggested that a modified process using intercritical annealing instead of full austenization annealing can be employed to produce the Q&P steel.[5] Such a Q&P steel consists of ferrite, martensite, and retained austenite. Intercritical annealing is now widely used in the steel industry to produce Q&P steel with the tensile strength of 1000 MPa. Nevertheless, the difference between the industrially produced Q&P steel fabricated by a full austenization annealing and intercritical annealing has not been fully investigated to date, especially the difference regarding the microstructure, mechanical properties, and deformation mechanisms. The current study aims to investigate the difference in the Q&P steel produced industrially by the full austenization annealing and the intercritical annealing
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