Microstructure, Mechanical Properties, and Toughening Mechanisms of a New Hot Stamping-Bake Toughening Steel
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NTRODUCTION
THE rapidly increasing number of vehicles across the globe brings many problems such as atmospheric pollution and energy crisis in recent years. Making use of stronger automotive components is a solution to build automobiles because parts with higher strength can be thinner and lighter.[1] By now, hot stamping is the only established manufacturing process for such automotive parts with strengths higher than 1200 MPa. In hot stamping, a blank usually made of boron steel is heated up into a fully austenite condition, then it is formed and quenched in closed dies into full martensite. 22MnB5 is a commonly used steel grade for this process. The microstructure of the material is ferrite/pearlite at room temperature, and it transforms into full martensite after hot stamping. Accordingly, the tensile strength rises from 600 MPa initially to approximately 1500 MPa.[2] Such ultrahigh strength allows a reduction in sheet thickness, but it does not have a direct contribution to an increase of crash safety or absorbed crash energy, which is determined by the product of strength and elongation (PSE) of the hot-stamped part. Currently, the ductility of the hot-formed steel, e.g., 22MnB5, is approximately 6 pct[3] after hot stamping. TAO LIN, Ph.D. Student, HONG-WU SONG, MING CHENG, and YUN CHEN, Associate Professors, and SHI-HONG ZHANG, Professor, are with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P.R. China. Contact e-mail: shzhang@ imr.ac.cn WEI-JIE LIU, Professor, is with the RIST, Northeastern University, Shenyang 110819, P.R. China. Manuscript submitted June 4, 2014. Article published online July 8, 2015 4038—VOLUME 46A, SEPTEMBER 2015
This means a relatively low PSE level of 9 GPa pct, which is far less than 20 GPa pct. So the hot-formed steel becomes insufficient for making vehicle components with good crashworthy qualities. To enhance the PSE level of steel, quenching and partitioning (Q&P) steel has been developed. The corresponding process is the Q&P process, which was first proposed by Speer.[4,5] The microstructure of the material is composed of a martensitic matrix and carbon-enriched retained austenite after the Q&P process. The martensite provides ultrahigh strength and the retained austenite provides good elongation up to 15 pct.[4] To produce such mixed microstructures, the Q&P steel is first austenitized and then quenched to a temperature (quenching temperature, TQ) between the martensite start (Ms) and finish (Mf) temperatures, so that specific fraction of retained austenite is obtained. The retained austenite is kept at a constant temperature (partitioning temperature, TP) for a certain time to stabilize the retained austenite by carbon diffusion from the carbon-supersaturated martensite to the neighboring austenite. Finally, the steel is quenched to room temperature and the retained austenite remains metastable, whereas the rest transforms into martensite. Many studies have been conducted to investigate the effect of the parameters of the Q&P process,[6–14] such as qu
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