Understanding and Avoiding Intergranular Fracture Characteristics of Hadfield/Hot-Press-Forming Multi-Layer Steel Sheets
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RECENTLY, multi-layered steel (MLS) sheets have been developed in automotive and steel industries to overcome limitations of monolithic steels having a trade-off relationship between strength and ductility.[1–4] Various combinations of materials and processing methods have been utilized for the MLS development. Koseki et al.[5–7] reported a good strength–ductility combination in MLS sheets composed of martensitic and stainless steels. Inoue et al.[8,9] achieved a high tensile strength of about 1200 MPa with a moderate uniform elongation of 15 to 20 pct by fabricating MLS sheets composed of martensitic and austenitic stainless
MIN CHEOL JO, HYEOK JAE JEONG, HYOUNG SEOP KIM and SUNGHAK LEE are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 790784, Korea. Contact e-mail: [email protected] JAEYEONG PARK is with the Center for Materials and Energy Measurement, Korea Research Institute of Standards and Science, Daejeon 34113, Korea. GYEONGBAE PARK and NACK J. KIM are with the Graduate Institute of Ferrous Technology, Pohang University of Science and Technology, Pohang 37673, Korea. TAEJIN SONG is with the Sheet Products & Process Research Group, Technical Research Laboratories, POSCO, Kwangyang 545-090, Korea. Manuscript submitted November 02, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
steels, and multi-layer TWIP/TRIP steels exhibiting far beyond the estimate values of rule of mixture was reported by Zhang et al.[10] As a promising candidate MLS material, austenitic TWinning-Induced Plasticity (TWIP) steels having highly sustained strain hardening as well as excellent tensile strength and ductility have been suggested. Their yield strength is relatively low at 200 to 600 MPa because of inherent characteristics of austenite,[11,12] and are often subjected to the delayed fracture,[13] liquid metal embrittlement,[14,15] and deterioration of surface quality.[16] In order to overcome these shortcomings, MLS sheets composed of TWIP steel and high-yieldstrength martensitic steels have been developed.[17–19] Chin et al.[17] fabricated a TWIP steel-cored MLS sheet covered with two martensitic hot-press-forming (HPF) steel layers by a hot-rolling process to have a high yield strength of 1073 MPa and a good ductility of 32.4 pct. Excellent yield strength (735 MPa) and uniform elongation (25.7 pct) could also be achieved from MLS sheets composed of martensitic steel layers and TWIP or stainless steel layers.[18] In TWIP/maraging MLS sheets, the strength and ductility could reach 1527 MPa and 21.5 pct, respectively.[19] However, the ductility of the MLS sheets is often seriously deteriorated by the rapid delamination or failure[20–22] as intergranular cracks appeared at
interfacial areas.[21,23,24] It is well known that the intergranular cracking occurring during the heat treatments after the hot-roll bonding is mainly attributed to the formation of precipitates or impurity segregation along coarsened prior austenite grain boundaries,[23–26] but very few lines of
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