Characterization and Control of the Compromise Between Tensile Properties and Fracture Toughness in a Quenched and Parti
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INTRODUCTION
SINCE the first proposal of quenched & partitioned (Q & P) process by Speer et al.,[1] the family of Q & P steels, which belongs to the third generation of advanced high-strength steels (AHSSs), has received major attention in particular regarding the understanding of the microstructure evolution and the improvement of mechanical properties.[2] In addition, the Q & P process has been successfully applied in the conventional industrial steel processes such as for AISI 430-type stainless steel,[3] austempered ductile cast iron,[4] medium carbon bar steels,[5] and hot stamping steels.[6] The Q & P process involves a quenching stage from the austenite or
ZHIPING XIONG, PASCAL J. JACQUES, and THOMAS PARDOEN are with the Universite´ catholique de Louvain, Institute of Mechanics, Materials and Civil Engineering, IMAP, 1348 Louvainla-Neuve, Belgium. Contact e-mail: [email protected]; [email protected] ASTRID PERLADE is with the ArcelorMittal Global R&D Maizie`res Products, Voie Romaine, BP 30320, 57283 Maizie`res-le`s-Metz Cedex, France. Manuscript submitted January 6, 2019. Article published online May 17, 2019 3502—VOLUME 50A, AUGUST 2019
austenite and ferrite domain where martensite forms and a following partitioning stage where carbon redistributes between martensite and austenite,[7] leading to the retention of some amount of austenite at room temperature named retained austenite (RA).[5,8] Q & P steels combine high ultimate tensile strength (UTS) and large ductility owing to the combined presence of tempered martensite and RA as demonstrated by many works focusing on the tensile properties.[9–11] Tempered martensite provides high strength, while the presence of RA improves ductility through enhanced progressive work hardening resulting from the transformation-induced plasticity (TRIP) effect. The amount of RA can be controlled via the adjustment of the quenching temperature and partitioning time at different partitioning temperatures.[7,9–11] In turn, the tensile properties are adjustable by tuning the amount, morphology, and stability of RA, leading to a wide range of properties.[12,13] Regarding the cracking resistance of Q & P steels and other steels in general, the tendency in the literature is often to use the product of UTS and total elongation (TE) as a fracture indicator. However, this link with fracture toughness and tensile properties can be misleading. For AHSSs, higher ductility does not
METALLURGICAL AND MATERIALS TRANSACTIONS A
necessarily mean higher flangeability as for other ductile steels.[14] For instance, although DP 1000 steel (ferrite-martensite Dual-Phase steel having a UTS of ~ 1000 MPa) has a larger TE than CP 1000 steel (Complex-Phase steel having a UTS of ~ 1000 MPa mainly consists of bainite and martensite), DP 1000 steel shows a smaller hole expansion ability because of a higher work hardening in the DP steel due to soft ferrite and hard martensite, and a lower work hardening in the CP steel due to the compatible deformation between bainite and martensite.[15] Ed
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