Medium-Mn Martensitic Steel Ductilized by Baking
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INTRODUCTION
PRESS-HARDENING steels (PHS) have been used in structural design of car bodies for anti-intrusion and crash energy management applications for more than 30 years.[1] The production of PHS components involves the hot-stamping process, including the heating and press-forming in the fully austenitic region and subsequent in-die quenching for a fully martensitic microstructure. The martensitic grade of 22MnB5 is the most widely used PHS due to the sufficient combination of strength and ductility/toughness.[2] However, the new wave of fuel-economy regulations demands the development of novel PHS with better combination of strength and ductility/toughness, so that the stringent design requirements can be met. Martensitic microstructure is the basis of many high-strength steels and has been studied extensively.[3–11] The high strength of martensitic microstructure mainly arises from a combination of high defect density with interstitial solution hardening, and carbon content is known as the most important metallurgical factor in setting the strength level.[3,7] However, higher
Q. LU, J. WANG, and H. ZHAN are with the China Science Lab, General Motors Global Research and Development, Shanghai 201206, China. Contact e-mail: [email protected] M. EIZADJOU, A. CEGUERRA, and S. RINGER are with the Australian Centre for Microscopy & Microanalysis, and School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW 2006, Australia. L. WANG is with the Research and Development Center, Baoshan Iron and Steel Co. Ltd., Shanghai, China. Q. LAI is with the Herbert Gleiter Institute of Nanoscience, Nanjing University of Science and Technology, Nanjing, China. Contact e-mail: [email protected] Manuscript submitted December 18, 2018. Article published online July 2, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
carbon level would dramatically deteriorate the ductility/toughness and weldability,[12] and proper heat treatment is usually necessary to improve the mechanical properties in carbon-based martensitic steels. It is well established that the ductility of the carbon-based martensitic steels can be improved by tempering through controlled carbide precipitation[3,8] at the cost of reduced strength. Recently, the quenching and partitioning (Q&P) in silicon-containing steels exhibit the potential in improving ductility via stabilizing retained austenite (RA) during the steelmaking process.[13,14] But the implementation of Q&P into industrial hot-forming processes is challenging in that the partitioning requires a second furnace and adds cycle time. A new grade of PHS is recently developed involving medium manganese content and low carbon level (0.14C-7.0Mn), with the hope of achieving a high strength–ductility/toughness synergy. The relatively high manganese content was designed for multiple purposes. Firstly, a lower soaking temperature is allowed, so that the prior austenite grain size (PAGS) is refined in the absence of micro-alloying elements.[15] Secondly, increasing Mn content results in a decre
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