Application of Quenching and Partitioning (Q&P) Processing to Press Hardening Steel
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INTRODUCTION
THE use of press hardening steel (PHS) in structural safety-related parts has experienced a rapid growth in the automotive industry, due to increased passenger safety standards which require an improved vehicle intrusion resistance.[1,2] Press hardening, also known as hot press forming, hot stamping or die quenching, is a technology to produce automotive ultra-high strength steel (UHSS) parts without spring-back. The hot stamping process can be carried out using two different methods: the direct and the indirect hot stamping method. In the direct hot stamping process, a blank is heated up in a furnace, transferred to the press, and subsequently simultaneously press-formed and quenched in water cooled dies. In the indirect method, the steel sheet is deformed to 90 to 95 pct of its final shape in a forming press with conventional dies prior to austenitization and final die quenching. The indirect method makes it easier to produce parts with larger dimensions and more complicated shapes. The indirect method, which may include several pre-forming presses, is
EUN JUNG SEO and LAWRENCE CHO, Graduate Students, are with the Graduate Institute of Ferrous Metallurgy, Pohang University of Science and Technology, Pohang 790-784, South Korea. BRUNO CHARLES DE COOMAN, Professor, is with the Graduate Institute of Ferrous Metallurgy, Pohang University of Science and Technology, and also Director, with the Materials Design Laboratory, Pohang University of Science and Technology. Contact e-mail: decooman@ postech.ac.kr Manuscript submitted November 26, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
therefore becoming more widespread.[1] In recent years, PHS has increasingly been used in automotive car bodies due to its specific advantages, such as an absence of spring-back-related shape effects and the achievement of an ultra-high strength of approximately 1500 MPa. PHS parts are mainly passenger safety-related anti-intrusion parts and structural reinforcements, such as door beams, impact beams, bumpers, pillars, roof rails, and tunnels.[3] The requirement of an ultra-high strength has, however, lead to concerns about the in-service properties such as fatigue, residual bendability, and toughness. Quenching and Partitioning (Q&P) processing is a method of stabilizing austenite in steel by quenching the steel to produce a specific volume fraction of martensite and subsequently partitioning the C from martensite to austenite during the partitioning stage. Q&P processing of advanced high strength steel (AHSS) has been shown to improve the ductility of AHSS.[4] By applying the Q&P processing concept to PHS, the ductility of PHS parts is expected to improve significantly without degradation of the strength. The Q&P-processed PHS might also be applied to parts where additional inservice bending performance is required to avoid catastrophic brittle fracture in passenger car collisions. The carbide-free bainite microstructure is recognized as a possible constituent for Q&P-processed steel, because the formation of carbide-free b
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