Liquid-Metal-Induced Embrittlement of Zn-Coated Hot Stamping Steel
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INTRODUCTION
THE importance of the automobile weight reduction by using high-strength steel sheets has increased remarkably, and this has led to an improvement in fuel efficiency and passenger safety.[1,2] Mainly cold-formable advance high-strength steels (AHSS) such as dualphase steel and transformation-induced plasticity steel are used for car body structural parts. The stamping of AHSS is challenging, because the use of high-strength steels makes the stamped parts very sensitive to springback if no special precautions are taken.[3] Hot stamping is an efficient process to produce ultra-high-strength steel parts with good dimensional accuracy. The steel sheets are heated and simultaneously press-formed and quenched in water-cooled dies to obtain a high-strength fully martensitic microstructure. The hot-stamped parts are not sensitive to springback.[4,5] Because of this advantage, the production of hot-stamped car parts has drastically increased in recent years, with the number of industrially produced parts reaching approximately 107 million in 2007.[6] The global demand for presshardened components for body-in-white application is expected to increase to 650 million parts in 2015.[7] A further illustration of the impact of hot stamping is the fact that more than 100 hot stamping production lines had been built globally by 2009 to address the strong demand from automotive companies.[8] CHANG WOOK LEE, Graduate Student, SEOK-JAE LEE, Research Assistant Professor, and BRUNO C. DE COOMAN, Professor, are with the Graduate Institute of Ferrous Technology (GIFT), POSTECH, Pohang, 790-784, Korea. Contact e-mail: decooman@ postech.ac.kr DONG WEI FAN, formerly Postdoctoral Researcher, Graduate Institute of Ferrous Technology (GIFT), POSTECH, is now Researcher, ArcelorMittal Global R&D, 3001 E, Columbus Drive, East Chicago, IN 46312. IL RYOUNG SOHN, Principal Researcher, is with the Technical Research Laboratories, POSCO, Gwangyang, 545-090, Korea. Manuscript submitted November 1, 2011. Article published online September 5, 2012 5122—VOLUME 43A, DECEMBER 2012
The 22MnB5 steel grade is a boron-added C-Mn steel widely used for hot stamping. The alloying elements C, Mn, and B retard the ferrite formation and the austenite decomposition during cooling. This enhanced hardenability facilitates the formation of martensite at relatively slow cooling rates.[9] Different types of coatings have been developed for hot-stamped steels to improve the in-service corrosion resistance of the hot-stamped parts. Aluminized coatings consisting of a near eutectic Al-10 pct Si alloy make up the most extensively used coating system.[10] The thin Al-10 pct Si alloy (~25 lm) protects the steel against decarburization and surface oxidation during the high-temperature thermal cycle required for the hot stamping. In addition, the integrity of the coating layer is guaranteed at the high working temperatures used in hot stamping due to the high melting temperature of Fe-Al alloys.[5,11] However, cracks are generated by the deformation during stamping step,[10
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