Room-Temperature Quenching and Partitioning Steel
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For the essential role of steels played in the automotive industry, their high strength is desirable in designing the lightweight structural components in terms of reducing greenhouse gas emission and improving fuel efficiency.[1] Unfortunately, increasing strength often leads to a decrease in ductility, which is known as the strength–ductility tradeoff.[2] This dilemma can be circumvented by engineering defects[3–5] or by creating macroscopically inhomogeneous structure[2,6] in steels. For instance, a deformed and partitioned (D&P) steel with an extremely high yield strength of 2.21 GPa and large uniform elongation of 16 pct has recently been developed by engineering dislocations.[3,5] The high dislocation density, which is mainly responsible for the ultrahigh yield strength and high ductility in this D&P steel, is achieved by multiple deformation processes.[3] Different from the previous efforts,[2,3,5,6] the current study aims to develop strong and ductile steels using a simple heat treatment. Steels have different kinds of phase transformation which results in transformed products with different
B.B. HE, L. LIU, and M.X. HUANG are with the Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, 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 March 29, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
mechanical properties.[7] By combining soft ferrite matrix and hard martensite islands using a single heat treatment (i.e., intercritical annealing), the dual-phase (DP) steel has opened up a new era for weight reduction in automotive industries around 40 years ago.[8,9] The ferrite phase is formed during the intercritical annealing, while the dislocated martensite phase is introduced during the quenching process down to the room temperature.[8] However, the soft ferrite matrix limits the strength of DP steel.[10] Although it is effective to increase martensite fraction to improve the strength of the DP steel, the corresponding strength and ductility could be subjected to undesirable tradeoff.[9] The other thermal processing routes adopted to produce martensite matrix in most strong and ductile steels are complicated compared to that for DP steel. For example, to achieve the best combination of strength and ductility, the proper quenching temperature to achieve martensite matrix in a quenching and partitioning (Q&P) steel should be precisely controlled, and such quenching temperature is deviated from room temperature,[11–13] making the steel processing difficult and the mechanical properties less stable. AQDespite intensive research efforts on the Q&P process and its variants over the last 10 years,[14–19] the complication in the processing route resulting from high-temperature quenching has not yet been resolved, leading to high industrial production threshold of Q&P steel. Inspired by the simplicity of room-temperature quenching to achieve martensite in DP steel,
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