Evolution of Microstructure and Mechanical Properties of Thermomechanically Processed Ultrahigh-Strength Steel
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INTRODUCTION
DURING the last few decades, various types of low carbon structural steel plates and line pipe steels were developed by the extensive use of thermomechanical controlled processing (TMCP),[1,2] which has become the most powerful and effective manufacturing process to satisfy increased hardenability,[3,4] improved strength, and superior low-temperature toughness.[5] Alloying elements such as 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 such as Ti and Nb, which promote the formation of carbides and carbonitrides, are also added to achieve the grain refinement of austenite. Several types of precipitates were reported to be formed at different stages of processing of low carbon microalloyed steel.[6] While coarse TiN particles of up to 3 lm in size are formed during or soon after casting, Ti-rich and Nb-rich precipitates (30 to 300 nm) form during rolling. Niobium carbide (10 nm) precipitates during austenite transformation and very fine Nb-rich precipitates (£5 nm) are formed after cooling. In the mid-1980s, the United States Navy developed low carbon Cu bearing high-strength low-alloy (HSLA) steels such as HSLA-80 and HSLA-100 as replacements for HY-80 and HY-100 steels, respectively.[7] Earlier the HY-130 type of steels were quenched and tempered to achieve strength and toughness, but welding of these steels is difficult and expensive. Recent development in P.S. BANDYOPADHYAY, Director, is with the Ordnance Development Centre, Metal and Steel Factory, Ishapore 743 144, India. S.K. GHOSH, Associate Professor, S. KUNDU, Lecturer, 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 October 9, 2010. Article published online April 30, 2011 2742—VOLUME 42A, SEPTEMBER 2011
the area of high performance steel (HPS)[8] with lower alloying additions has reduced cost due to the elimination of preheating during welding by lowering the carbon equivalent. This 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, which have been commonly produced in accordance with the military specification (MIL-S-24371B),[9] find applications in heavy section, particularly for producing submarine hull plate and hydrofoils. However, current research has been directed toward further progress in this area to meet greater challenges. In this context, the limitations of conventional ferrite-pearlite steels such as low ductility, poor toughness, and weldability were realized, which has triggered a paradigm shift toward multiphase microstructures, comprising ferrite and lowtemperature transformation products such as bainite, martensite, and some amount of retained austenite. The preceding breakthrough has resulted in evoluti
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