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for high-strength structural applications are essentially quenched and tempered low- to mediumcarbon low-alloy steels. In the United States Navy’s specification, these are basically HY series of steels that are primarily low-alloy Ni-Cr-Mo-V steel with about 0.2 pct C (Tables I and II). The HY series of steels has been most commonly used for submarine hulls for over 4 decades.[1] In this series of steels, the initial development of the HY-80 was followed by the HY-100 and the HY-130 steels (the number indicates yield strength (YS) in Ksi; 1 Ksi ⫽ 7 MPa). Carbon is the main martensitic strengthening element in these steels; with a carbon equivalent of around 0.80, the steels are difficult to weld and require costly preheating as well as postweld heat treatments.[2,3] To overcome these difficulties, a new family of low-carbon, copper-bearing, high-strength, lowalloy steels has been developed during the last 2 decades (Tables I and II). These steels are intended for use in the construction of the hulls of naval warships and submarines and also for engineering structures, especially mining and dredging equipment, offshore drilling platforms, heavy duty trucks, and bridges, etc.[1,3–5] Using high strength, good impact toughness, and easy weldability as the major criteria, low-alloy steels with a very low carbon content (⬍0.06 pct) and a combination of alloying elements (Cu, Ni, Cr, Mo, Nb, and V) were developed. Copper was added primarily to cause precipitation hardening. The first successful development in this family of steels was an ASTM A 710 grade with a composition 0.07 pct C, 0.5 pct Mn, 0.4 pct Si, 0.75 pct Cr, 0.9 pct Ni, 0.20 pct Mo, 1.15 pct Cu, and 0.02 pct S.K. DHUA, Principal Research Manager, and D. MUKERJEE, Deputy General Manager, are with the Research and Development Centre for Iron and Steel, Steel Authority of India Limited, Ranchi - 834 002, India. D.S. SARMA, Professor, is with the Department of Metallurgical Engineering, Banaras Hindu University, Varanasi - 221 005, India. Manuscript submitted May 16, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A

Nb for use in offshore structures.[6–11] Based on this, HSLA80 steel was developed by the United States Navy in the early 1980s.[1,12] Thomson et al. worked with a similar HSLA steel with a higher alloy modification.[13] More recently, they worked in detail on the austenite decomposition during continuous cooling on an HSLA-80 plate steel.[14] Although HSLA-80 steel has a minimum YS of 80 Ksi (552 MPa) with a good low-temperature impact toughness (81 J at ⫺85 ⬚C) and weldability, there was a need to develop steels with a still higher YS for making many critical components of naval and engineering structures that were subjected to complex dynamic loading.[1] With this in mind, a few United States steel companies (Lukens Steel, Coatesville, Pennsylvania, Phoenix Steel Corporation, Claymont, Delaware, etc.) and the United States Navy have concentrated on the development of HSLA-100 steel with a minimum YS of 100 Ksi (690 MPa) with a low-temperature impact