An Ultra-low Carbon, Thermomechanically Controlled Processed Microalloyed Steel: Microstructure and Mechanical Propertie
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THE huge energy resources in the arctic region demand large diameter pipelines for oil and gas transportation through severe environments. Strength and low-temperature toughness, weldability, resistance to hydrogen-induced cracking, and resistance to fatigue were the critical properties required in these types of steels. The conventional ferrite-pearlite steels are not suitable to achieve the above properties. In response to the above demand, the evolution of high-strength lowalloy (HSLA) steel was based on low carbon content to improve weldability and suitable alloying elements were added to improve austenite hardenability. Alloying elements, e.g., Mn and Ni, were also added to reduce the impact transition temperature. Thermomechanical controlled processing (TMCP) and microalloying in order to obtain desired microstructure and properties are the essence of ultra-low carbon microalloyed steel. Efforts were undertaken to develop a low-cost, ultralow carbon steel with microalloying elements through TMCP. The carbon content in ultra-low carbon steel R. SHUKLA, formerly Assistant General Manager, with the Research & Development Center for Iron & Steel, SAIL, Ranchi 834002, India, is now Deputy General Manager with Steel Processing Unit, Bettiah, West Champaran, Bihar 845450, India. S.K. DAS and B. RAVI KUMAR, Principal Scientists, are with the Materials Science and Technology Division, CSIR, National Metallurgical Laboratory, Jamshedpur 831 007, India. S.K. GHOSH, Associate Professor, S. KUNDU, Assistant Professor, and S. CHATTERJEE, Professor, are with the Department of Metallurgy and Materials Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711103, India. Contact e-mail: [email protected] Manuscript submitted January 31, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS A
generally varies from 0.01 to 0.02 wt pct. A further decrease in carbon content improves weldability and corrosion resistance properties of these steels at the cost of strength and toughness.[1–3] Microalloying elements Ti, Nb, and V are incidentally strong carbide formers, and also Nb acts as grain refiner while in solution of austenite. On subsequent cooling, these microalloying elements precipitate as carbides and/or carbonitrides at the austenite grain boundary. Precipitation hardening together with grain refinement and dislocation strengthening are used to achieve the desired strength and toughness property. In recent times, these steels are being used in pipeline and automobile sectors due to its high toughness and good corrosion resistance along with its adequate strength for application. However, continuous efforts are devoted to further improve its strength and toughness along with good weldability through newer alloy design and processing techniques.[4–9] It needs to be mentioned that the modern steel technology demands microstructurally engineered excellent properties for manufacturing of pipelines and other structural applications. Electron backscatter diffraction (EBSD) is a microstructural-crystallographic
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