Development of High-Strength Cu-Ni-Ti-B Multiphase Steel by Direct Air Cooling
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ODUCTION
THE demand for maximizing strength-ductility combination of high-strength thin-gaged structural steels is mounting rapidly. In this regard, dual-phase steels with continuous work-hardening characteristics have drawn serious attention.[1] In such steels, sufficient hardenability of austenite to avoid pearlite formation during cooling can be achieved by partitioning of solutes from intercritical ferrite to austenite during batch or continuous annealing, which require extensive installations.[2] Microalloying with elements such as Ti and B also improve the hardenability of austenite.[3,4] It is also known that addition of Cu retards the austenite (c) fi ferrite (a) transformation.[5–7] In the present study, the necessary hardenability of austenite is achieved by microalloying (Ti, B) and Cu addition (1.5 wt pct) without using the partitioning effect during intercritical annealing with an aim to develop directly air-cooled low-carbon (0.035 to 0.055 wt pct) steels. The benefit of Cu addition has also been utilized in enhancing the mechanical properties by age hardening. Addition of Ni by an amount half of Cu is utilized to suppress the hot shortness.[8] To minimize the redundant experimental trials, artificial neural network (ANN) technique was employed to predict the tentative continuous cooling transformation behavior of the alloy. Although the details of the neural network modeling and results are elaborately discussed elsewhere,[9,10] it is imperative to briefly describe the model and some of the relevant results for conceptualization S.K. GHOSH, Assistant Professor, and P.P. CHATTOPADHYAY, Professor, Department of Metallurgy and Materials Engineering, and S. GANGULY, Doctoral Student, School of Materials Science and Engineering, are with the Bengal Engineering and Science University, Shibpur, Howrah-711 103, India. Contact e-mail: [email protected]. ac.in A. HALDAR, Researcher, is with the R & D Division, Tata Steel, Jamshedpur-831 001, India. Manuscript submitted May 8, 2007. Article published online July 30, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
of the experimental work. The influence of the cooling rate on austenite transformation, and hence the final microstructure, has been examined by dilatometric study using Gleeble 1500 thermomechanical simulator (Dynamic Systems Inc., Poestenkill, NY). The activation energy values for precipitation of Cu in different alloys are determined by Kissinger’s method[11] to understand the aging kinetics in such alloy. The workhardening behavior of the directly air-cooled samples has been analyzed by the differential Jaoult–Crussard (J-C) methods[12,13] before and after aging.
II.
EXPERIMENTAL
The alloys were prepared in a laboratory scale induction-melting furnace (5 kg capacity). The ingots were cast into a preheated cast iron mold with approximately 50-mm-square section. Table I presents the compositions of the alloys obtained by spectroscopic analysis using an optical emission spectrometer (Spectrolab-M8, Spectro Analytical Instruments, Kleve, Germany). The cast i
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