Development of Fine-Grained, Low-Carbon Bainitic Steels with High Strength and Toughness Produced Through the Convention
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TRODUCTION
BAINITE is an important microstructural constituent for a wide range of commercial steels. It is well known that bainitic structures can provide many beneficial properties to steels.[1–13] For example, bainitic structures can give better creep resistance to steels in comparison to ferrite/pearlite structures.[14,15] Its property of environmental resistance under certain conditions is better than that of the conventional steel structures, particularly in the case of hydrogen embrittlement, corrosion fatigue, and in few cases of rolling contact fatigue.[16] High-strength bainitic steel plates can be a good candidate for use in machinery, buildings, and heavy equipment for construction, which requires high strength as well as high toughness. It can be further applied to line pipe applications and offshore constructions requiring excellent low-temperature toughness. Grain size is an important parameter for strength and toughness of steels. Fine grain size is always desirable for achieving good strength with improved toughness in steels. Therefore, strengthening of steel by grain size reduction has always been a good choice among the SANJAY KUMAR DHUA, Deputy General Manager, PARTHA PRATIM SARKAR, Senior Manager, ATUL SAXENA, General Manager, and BIMAL KUMAR JHA, Former Executive Director, are with the R&D Centre for Iron & Steel, Steel Authority of India Limited, Ranchi, Jharkhand, 834002, India. Contact e-mails: [email protected], [email protected] Manuscript submitted December 5, 2015. Article published online September 13, 2016 6224—VOLUME 47A, DECEMBER 2016
researchers.[2,8,17] This basic concept has also led to the development of thermomechanical-controlled processing capable of refining the prior austenite grains before their transformation. These refined austenite grains lead to correspondingly refined microstructures.[18,19] The microalloying elements like niobium and titanium are essentially added to prevent grain growth during the hot-rolling process. These microalloying elements have nominal solubility in austenite and need to be added in small quantity to form carbides or carbonitrides that prevent austenite grain coarsening during hot deformation and subsequent cooling. Among these microalloying elements, titanium is known to suppress austenite grain coarsening more effectively at elevated temperatures by formation of stable TiN precipitates. TiN particles of size
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