Influence of martensite content and morphology on tensile and impact properties of high-martensite dual-phase steels
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INTRODUCTION
DUAL-PHASE (DP) steels have a composite microstructure of martensite and ferrite and exhibit a good combination of strength and ductility and a high work-hardening rate. Most of the research work on DP steels conducted so far was directed toward understanding the role of chemistry (primarily, variations in C, Mn, Si, and V) and microstructural variables on the steel’s tensile and formability characteristics.[1,2,3] It is now established that the microstructural parameters of significance are the volume fraction, size, and distribution of the constituent phases. However, most of the research work conducted to date has been focused on microstructures containing a volume fraction of martensite (Vm) less than 0.25.[2,3] The lack of research interest in highVm DP steels can be attributed to the earlier observation that the ductility and impact toughness of these materials degrade rapidly with increasing martensite content above 0.25.[4] The degradation of ductility and impact toughness of highVm-containing DP steels has been attributed to the formation of coarse martensite phases. This observation suggests that it may be possible to improve the ductility and toughness by developing microstructures with very fine grains and a uniform distribution of ferrite and martensite phases. Dualphase steels containing such microstructures are obtained in this work by adopting suitable heat-treatment procedures. The present investigation examines the tensile and impact properties of these steels and compares them to those of conventionally processed DP steel containing coarse or A. BAG, formerly Manager and Head, Materials Science Laboratory, R&D Centre, Bharat Earth Movers Limited, Kolar Gold Fields, 563115 India, is with the School of Mechanical and Production Engineering, Nanyang Technological University, Singapore 639798. K.K. RAY, Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur - 721 302, India. E. S. DWARAKADASA, Professor, is with the Department of Metallurgy, Indian Institute of Science, Bangalore - 560 012, India. Manuscript submitted July 7, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
banded martensite. In particular, this work focuses on understanding the tensile and impact properties of high-martensite (.0.25) DP steels. II. EXPERIMENTAL PROCEDURE A microalloyed steel supplied by Swedish Steel (Oxelosund, Sweden) was selected as the starting material for making DP microstructures. The as-received steel was in the form of 14-mm-thick hot-rolled plates in a quenched and tempered condition. The chemical composition of the steel, determined using various chemical analysis techniques, is shown in Table I. Specimen blanks, 210 3 70 3 14 mm in size, were subjected to either intermediate quench (IQ) or step quench (SQ) heat-treatment schedules. The IQ treatment consisted of a double quench operation; the specimens were first soaked at 920 8C for 30 minutes and were quenched in a 9 pct iced brine solution (27 8C). These were then held at di
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