Microstructure, properties, and age hardening behavior of a thermomechanically processed ultralow-carbon Cu-bearing high
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I. INTRODUCTION
RECENT studies on high-strength low-alloy (HSLA) steels have been focused on improving the weldability and corrosion resistance without impairing strength and toughness. Hence, newer alloy designs and various processing techniques have evolved to meet such requirements.[1–6] In 1980, HSLA-80 and HSLA-100 steels were developed by the United States Navy as a replacement for the HY-80 and HY-100 steels, respectively, which are low-carbon, Cu-precipitation-strengthened steels.[7] These low-carbon, Cu-bearing HSLA steels are now attracting gradual interest as structural steels and are being produced on a commercial scale. Fine -Cu particles, precipitated during the aging process, impart high strength to such steels. Better lowtemperature toughness and weldability results from a very low carbon content. Besides high strength and toughness, these steels exhibit good resistance to hydrogen-induced cracking and excellent corrosion resistance properties, especially in a chloride atmosphere.[8,9] The Age-hardening behavior of Cu-bearing HSLA steels has been reported in the literature, and the carbon content of these steels is in the range of 0.04 to 0.06 wt pct.[10–16] By reducing the carbon content to an extremely low level in HSLA steels, weldability and toughness can be improved. However, studies on the structure and properties of ultralow-carbon HSLA steels have not been reported much in the literature. Previous works on ultralow-carbon Cu-bearing HSLA forgings showed that A. GHOSH and S. CHATTERJEE are with the Department of Metallurgy and Materials Engineering, Bengal Engineering College (Deemed University), Howrah-711 103, W.B., India. B. MISHRA is with the Metallurgical and Materials Engineering Department, Colorado School of Mines, Golden, CO 80401. Contact e-mail: [email protected] S. DAS is with the National Metallurgical Laboratory, Jamshedpur-831 007, India. Manuscript submitted April 20, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
a substantial amount of strength, together with high toughness at ambient and subambient temperatures, can be obtained by thermomechanical-controlled processing, and the variation in postcooling techniques engenders a variety of microstructures.[6,17,18] However, the age-hardening behavior of these steels has not been reported. Moreover, studies on the effect of finish rolling temperatures (FRTs) on the structure and properties of ultralow-carbon, Cu-bearing highstrength steel is scanty in the literature. In the present study, an ultralow-carbon, Cu-bearing steel has been prepared and subjected to a three-stage controlled rolling technique and, subsequently, cooled in water. The microstructures and associated mechanical properties at various processing conditions were investigated. The aging characteristics of the thermomechanical-control–processed (TMCP) steels plates have also been studied. II. EXPERIMENTAL PROCEDURE The steel composition given in Table I was prepared in an air induction furnace. The cropped ingot was hot forged to 16-mm-thick slabs and reheat
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