Effects of Shape and Orientation of MnS on Charpy Impact and Bending Properties in Hot-Press-Forming (HPF) Steels
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INTRODUCTION
GIGA-GRADE strength and excellent ductility are generally required properties in the automotive reinforcement parts.[1–4] These Giga-grade steel market shares are generally divided between high-strength cold-rolled sheets and hot-press-forming (HPF) sheets which are rapidly cooled after high-temperature austenitization and forming process to obtain a martensitic structure.[5–11] Recently, wide applications of HPF steel sheets have been limited by the reduction in Charpy impact toughness and formability, while achieving Giga-grade strengths, and also by the large energy consumption during the heating process. Thus, many studies on improvement of impact toughness and formability by the refinement of prior austenite grains,[12–15] changes in microstructural concepts and
MIN CHEOL JO, SELIM KIM, and SUNGHAK LEE are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea. Contact e-mail: [email protected] SEONGWOO KIM, JINKEUN OH, and IN SIK SUH are with the Steel Products Research Group, Technical Research Laboratories, POSCO, Kwangyang 545-875, Republic of Korea. Manuscript submitted October 29, 2018. Article published online February 6, 2019 1672—VOLUME 50A, APRIL 2019
heating processes,[16–21] or reduction in carbon content[22,23] have been actively conducted. The HPF steels might contain a small amount of inclusions, typical example of which is MnS particle elongated during the rolling, and their brittle characteristics and stringer shape can deteriorate the impact toughness and formability.[24–28] Since MnS particles can be controlled by minimizing the S content in the steel-making process, their existence has been recently ignored.[29–31] However, even a very small amount of MnS might act as a trouble-maker reducing the toughness and formability which are important properties in HPF sheet applications. The deterioration of toughness and formability by the existence of MnS is reasonably expected, but the verification of the reasons and related mechanisms of this deterioration by quantitative microstructural analyses has not been sufficiently made yet. In this study, effects of shape and orientation of MnS particles on Charpy impact and bending properties were investigated by conducting ex situ three-point bending or tension tests coupled with scanning electron microscope (SEM) observation. Deformation and fracture mechanisms of MnS particles were verified by the morphological change occurring at the same observation area, and the critical strain required for initiating the cracking or interfacial debonding of MnS particles was defined for understanding microstructural
METALLURGICAL AND MATERIALS TRANSACTIONS A
characteristics and for improving the impact toughness and formability of the HPF steels.
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EXPERIMENTAL
A. Ultra-High-Strength HPF Steels The HPF steels used in this study were commercial 1470-MPa-grade 1.4-mm-thick steels whose compositions were Fe-(0.15 to 0.25)C-(0.2 to 0.3)Si-(1.2 to 1.6)Mn-(0.0026 or 0.0009)S-(
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