Structure and Properties of a Low-Carbon, Microalloyed, Ultra-High-Strength Steel
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THE thermomechanical controlled processing (TMCP) composed of controlled rolling and accelerated cooling provides a useful technique for the microstructural control of hot-rolled steel plates. During the last four decades, various types of structural steel plates and line pipe steels were developed by the intensive use of TMCP and direct quenching (DQ) processes.[1,2] Various strengthening mechanisms such as grain refinement, precipitation hardening, and transformation strengthening mechanisms have been used in the development of DQ steel plates. DQ and tempering became the most powerful and effective manufacturing process to satisfy increased hardenability,[3,4] improved strength, and superior low-temperature toughness in a plate thickness up to 100 mm.[5] In the mid-1980 s the U.S. Navy developed lowcarbon Cu-bearing high strength low alloy (HSLA) steels (HSLA-80 and HSLA-100) as a replacement of HY-80 and HY-100 steels.[6] Alloying elements like Mn, Ni, Cr, and Mo are added to achieve the hardenability of austenite in these low-carbon steels. In addition to those alloying elements, microalloying elements like Ti and Nb, which are principally carbide and carbonitride formers, are also added to achieve the grain refinement of austenite. P.S. BANDYOPADHYAY, Director, is with the Ordnance Development Centre, Metal and Steel Factory, Ishapore 743 144, India. S. KUNDU, Lecturer, S.K. GHOSH, Assistant Professor, and S. CHATTERJEE, Professor, are with the Department of Metallurgy and Materials Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711 103, India. Contact e-mail: skghosh@metal. becs.ac.in Manuscript submitted August 11, 2010. Article published online November 2, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
Previously, the HY-130 type of steels were quenched and tempered to achieve strength and toughness, but welding of these steels was difficult and expensive. Recent developments in the area of high performance steel[7] with lower alloying additions reduced costs by eliminating the preheat requirement during welding by lowering the carbon equivalent (CE), which is an attractive choice to replace HY-130 for higher strength along with better weldability, fracture toughness, ductility, and corrosion resistance. High-strength microalloyed steels were commonly produced in accordance with the military specification (MIL-S-24371B)[8] and are used particularly for producing submarine hull plate and hydrofoils. However, current researches have been directed toward subsequent progress in this area to meet the further challenges. In this context, the limitations of conventional ferrite-pearlite steels like low ductility, poor toughness, and poor weldability have been realized, which leads to a paradigm shift toward multiphase microstructures comprising ferrite and low-temperature transformation products like bainite and martensite, with some amount of retained austenite, which results in the evolution of new steels with improved properties. Several types of precipitates were reported to be formed at di
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