Application of a Model for Quenching and Partitioning in Hot Stamping of High-Strength Steel
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UCTION
IN recent years, the demand for safety and energy saving in the automotive industry has been continuously increasing.[1] The hot stamping process of advanced high-strength steels (AHSSs) can effectively reduce the weights of parts with only a small loss in strength.[2] Moreover, the hot stamping of traditional boron steels leads to fully martensitic parts with very low postforming plasticity, which evidently reduces the overall mechanical properties and the application of hot stamping parts.[3] The quenching and partitioning (Q&P) process is currently being investigated to replace the direct quench in traditional hot stamping with the aim of improving the postforming ductility of high-strength structural parts. There are two Q&P approaches: one-step and two-step.[4,5] This paper will focus on the two-step Q&P process because hot stamping can be more productive with this process. This Q&P process consists of
BIN ZHU, ZHUANG LIU, LIANG WANG, and YISHENG ZHANG are with the School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, Hubei, People’s Republic of China. Contact e-mail: wangliang@mail. hust.edu.cn YANAN WANG, and BERNARD ROLFE are with the School of Engineering, Deakin University, Geelong, VIC, Australia. Manuscript submitted August 17, 2017. Article published online January 31, 2018 1304—VOLUME 49A, APRIL 2018
quenching of the part to a temperature between Ms and Mf, and the part is then heated to a partitioning temperature and held for a number of seconds until carbon diffuses from the supersaturated martensite to austenite.[6] After partitioning, the part is quenched to room temperature, where a significant portion of austenite is stabilized.[7] The final martensite–austenite duplex microstructure exhibits high strength (about 1500 MPa) and much greater elongation (14.8 pct) than that of typical hot stamped boron steel (6.6 pct).[8] A physically based model is developed to describe the relationship between the microstructure and the mechanical properties of Q&P-processed steels.[9] Research on the mechanism of Q&P process started over a decade ago. J.G Speer developed a CCE (carbon-constrained equilibrium) model to describe the carbon transfer from martensite to austenite, in the absence of any competing reaction.[10–12] The model assumes that the a/c interface is immobile and the chemical potentials of carbon in martensite and austenite at the interface are equal. However, some authors have observed a significant movement of the a/c interface during partitioning.[13] Santofimia et al.[14] built a new model to quantify the interface migration and carbon diffusion during partitioning. The main difference between these two models is the consideration of the interface movement. In this paper, two-step Q&P process with hot stamping is studied experimentally using a hot stamping mold.
METALLURGICAL AND MATERIALS TRANSACTIONS A
The mold temperature is controlled by heating rods and temperature control system. Besides, Santofimia’s model is used to calculate the
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