Yield Strength Enhancement by Carbon Trapping in Ferrite of the Quenching and Partitioning Steel
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INTRODUCTION
THE quenching and partitioning (Q&P) process involves (1) austenite (c) quenching and transforming partially into martensite at a temperature (TQ) between martensite-start (MS) and martensite-finish (Mf) temperatures, followed by (2) carbon partitioning from supersaturated martensite into the surrounding austenite at temperature (TP) either at or above initial quenching temperature (TQ). The carbon partitions into austenite in QP steels during partitioning process since the addition of silicon prohibits cementite precipitation.[1–8] The Q&P applies to either for fully martensite steel or to the intercritical annealed duplex microstructure. In the latter case, a QP steel,[9] containing
P. CHEN is with The State Key Laboratory of Rolling and Automation, Northeastern University, Shenyang 110189, P.R. China and also with the Australian Centre for Microscopy & Microanalysis, The University of Sydney, Sydney, NSW 2006, Australia. G.D. WANG and H.L. YI are with The State Key Laboratory of Rolling and Automation, Northeastern University. Contact e-mail: hityihl@ 126.com A.V. CEGUERRA and S.P. RINGER are with the Australian Centre for Microscopy & Microanalysis, The University of Sydney and with the School of Aerospace, Mechanical & Mechatronic Engineering, The University of Sydney, Sydney, NSW 2006, Australia and also with the Australian Institute for Nanoscale Science and Technology, The University of Sydney, Sydney, NSW 2006, Australia. A.J. BREEN is with the Australian Centre for Microscopy & Microanalysis, The University of Sydney and also with the School of Aerospace, Mechanical & Mechatronic Engineering, The University of Sydney. X.C. XIONG, Q. LU, and J.F. WANG are with the China Science Lab, General Motors Global Research and Development, Shanghai 201206, P.R. China. Manuscript submitted June 7, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
approximately 40 vol pct of ferrite, has a better combination of strength and elongation, exhibiting 850 MPa of yield strength. How the ferrite-involved microstructure achieves such a high yield strength in this material is an attractive point and has not yet been discussed in prior literature. The quenching-tempering & partitioning (Q-T&P) concept was proposed to retain austenite by carbon partitioning but with a heat treatment that is the same as the conventional Q&T process in a low density steel with 3.5 wt pct aluminum addition.[10–13] The alloys are originally from the d-TRIP steels. We denote the d-alloys treated by Q-T&P as ‘d-QP steel.’ Since we separated the quenching and partitioning processes in Q-T&P, we can obtain comprehensive information from before and after partitioning. In this research, we aimed to understand the reason why the ferrite-involved microstructure achieves such a high yield strength. Carbon trapping in ferrite during the partitioning process has been proposed to contribute to the high yield strength. The d-QP steel was intercritically annealed and quenched to ambient temperature, to obtain the initial microstructure of ferrite, m
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