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10/7/03

4:35 PM

Page 2493

Effect of Cooling Rate on the As-Quenched Microstructure and Mechanical Properties of HSLA-100 Steel Plates S.K. DHUA, D. MUKERJEE, and D.S. SARMA The effect of cooling rate on the as-quenched microstructure and mechanical properties of a 14-mm-thick HSLA-100 steel using various cooling media such as brine, water, oil, air, and furnace has been studied. While quenching in brine, water, and oil resulted in lath martensite structures, the granular bainite and martensite-austenite (M-A) constituents were found in air- or furnace-cooled specimens. The average lath spacing increased slightly on decreasing the cooling rate (300 nm in brine-quenched specimen to 400 nm in oil-quenched specimen). The precipitates of Cu and Nb(C, N) were observed in all the quenching conditions except in the brine-quenched specimen. The as-quenched strength and toughness of the brine-, water-, and oil-quenched specimens were higher (yield strength: 894 to 997 MPa, ultimate tensile strength: 1119 to 1153 MPa, and Charpy V-notch energies: 65 to 70 J at
85 °C) than those of air- and furnace-cooled specimens (yield strength: 640 to 670 MPa, ultimate tensile strength: 944 to 1001 MPa, and Charpy V-notch energies: 10 to 20 J at
85 °C). For industrial production of HSLA-100 steel plates, oil or water quenching is recommended in lower thickness plates (25 mm). For production of thicker plates, however, water quenching is more suitable.

I. INTRODUCTION

LOW-CARBON copper-bearing high-strength low-alloy (HSLA) steels have been developed over the past 2 to 3 decades as an alternative to the traditionally used lowto medium-carbon quenched and tempered high yield steels for structural applications.[1–13] Copper imparts precipitation strengthening in these steels.[14] Because carbon is lower (C  0.07 wt pct) in these newly developed structural steels, they have better weldability[15] and hence less fabrication cost. The ASTM A710 steel was first to be developed in this series of Cu-bearing steels in the late 1970s with a composition of 0.07 wt pct C, 0.5 wt pct Mn, 0.4 wt pct Si, 0.75 wt pct Cr, 0.9 wt pct Ni, 0.20 wt pct Mo, 1.15 wt pct Cu, and 0.02 wt pct Nb.[1–10] Subsequently, based on this composition, HSLA-80 steel was developed by the United State’s Navy in the early 1980s for construction of naval ships and submarines by replacing the traditionally used HY-80 steel.[11,12,13] After successful development of HSLA80 steel, the United State’s Navy, in association with a few United State’s steel companies, attempted to develop HSLA100 steel with improved strength (yield strength 692 MPa, minimum) maintaining good low-temperature impact toughness (CVN energies 81 J at
85 °C) and weldability. A few industrial trial heats were made in this regard in the late 1980s and early 1990s.[16,17] Since then, there have been efforts to develop an indepth understanding of the structure-property correlations of this steel. Although there have been several investigations[6,17–25] in the past on the quenched and tempered HSLA-100 ste