Static and dynamic fracture toughness of a medium carbon microalloyed steel of three different microstructures
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I. INTRODUCTION
ON account of cost-effective processing, medium carbon microalloyed (MA) steels (also known as HSLA [high strength, low alloy] steels) are increasingly used instead of quenched and tempered (Q&T) steels in the automotive industry for noncritical applications. However, the inferior tensile properties and fracture toughness of MA steels restrict their use in critical applications.[1] The simultaneous attainment of high strength and high fracture toughness is one of the most difficult goals in the development of HSLA steels. A possible method is to develop mixed microstructures through controlled thermomechanical treatments, with a ductile phase appearing in a desired morphology (size, shape, and distribution), along with highly tempered martensite.[2] Recently, a two-step cooling and annealing (TSCA) treatment was developed to produce a multiphase ferrite-bainite-martensite (F-B-M) microstructure that displayed tensile properties similar to those of the Q&T steels. It was reported that a decrease in the final deformation temperature led to a finer grain size (beneficial for toughness) and also promoted the formation of ferrite.[3,4,5] The present investigation forms a part of an attempt to improve the tensile properties and the fatigue resistance S. SANKARAN, Research Associate, is with the Center for Structural and Functional Materials and Department of Chemical Engineering, University of Louisiana at Lafayette, Lafayette, LA, U.S.A. Contact e-mail: [email protected] G. MALAKONDAIAH, Scientist, is with the Defense Metallurgical Research Laboratory, Kanchanbagh, Hyderabad 500 258, India. GOUTHAMA, Assistant Professor, and S. SANGAL, Professor, are with the Department of Materials and Metallurgical Engineering, Indian Institute of Technology, Kanpur, India. K.A. PADMANABHAN, Jawaharlal Nehru Professor, is with the School of Physics, University of Hyderabad, Hyderabad 500 046, India. Manuscript submitted November 26, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
of the automotive grade microalloyed steel 38MnSiVS5 through thermomechanical processing to produce a F-B-M microstructure.[6–9] Here an attempt was made to optimize the parameters (finish forging temperature, quenching temperature, annealing temperature and time) of the TSCA treatment. The presence of a ductile ferrite phase in a high-strength bainite-martensite matrix is expected to lead to good fracture resistance. In martensitic and bainitic steels, plane fracture toughness, KIC, increases with an increase in the packet size of the laths.[10] When a steel displays a mixed microstructure of martensite and bainite, the shape and distribution of the second-phase bainite have significant effects on the fracture toughness. Lower bainite, which appears in an acicular form and partitions prior austenite grains, improves the fracture toughness significantly.[10] The steel used in the present investigation had a mixed microstructure of F-B-M with a mixed morphology in each microstructural unit. To the best of our knowledge, the fracture toug
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