Effect of Cooling Rate and Chemical Composition on Microstructure and Properties of Naturally Cooled Vanadium-Microalloy
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TRODUCTION
HIGH-STRENGTH low alloy (HSLA) steels containing vanadium as the microalloying element are widely used for construction, line pipe, pressure vessel, engineering, automobile, naval, and defense applications.[1] V(C,N) precipitates retard the recrystallization of hot-deformed austenite at lower deformation temperatures [below ~1273 K (1000 C)], which can help in the refinement of ferrite grain size.[1–5] The prime objective of V addition is, however, to achieve precipitation strengthening from fine-scale VC or V(C,N) precipitates.[1–5] VN precipitates further contribute to ferrite grain refinement by acting as the heterogeneous nucleation sites for intragranular formation of both polygonal ferrite grains and acicular ferrite grains.[6–8] Vanadium-microalloyed HSLA steels have long been used for flat-rolled products such as plates and sheets. Considering the above-mentioned beneficial effects, ANISH KARMAKAR, DEBALAY CHAKRABARTI, and RAHUL MITRA are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpurm, West Bengal, 721-302, India. Contact e-mail: [email protected] POOJA SAHU is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology, and also with the CSIRInstitute of Minerals and Materials Technology, Bhubaneswar, Odisha, 751-013, India. SUMAN NEOGY is with the Mechanical Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Mumbai, Maharashtra, 400-085, India. SUBRATA MUKHERJEE and SAURABH KUNDU are with Research and Development, Tata Steel, Jamshedpur, Jharkhand, 831-001, India. Manuscript submitted June 7, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
V-microalloying is extensively used in developing highstrength steel grades for long product applications such as bars, beams and sections for construction sector and load-bearing components such as crankshaft and connecting rod for automotive sector.[9–13] A typical V content in the range of 0.05 to 0.07 wt pct is used for such applications and a high-N level is maintained (0.01 to 0.03 wt pct). Now reinforcement bars, sections, and plates can be of different sizes. As those products are hot-rolled (or forged) and air-cooled, natural cooling takes place and the cooling rate varies with the product size. Smaller the thickness of the steel products, faster is the cooling rate experienced by those products after hot-deformation processing. For the effective utilization of V in steel, there is a need to study the effect of cooling rate (during natural air cooling) on microstructure and mechanical properties. That is the first objective of the present study. In general, the effect of cooling rate on microstructures and hardness of HSLA steel is studied by thermal or thermo-mechanical simulators, such as dilatometer and Gleeble simulator, using small-sized specimens.[14–17] However, the cooling inside a thermal simulator is controlled cooling or forced cooling. There is a difference between natural cooling that the steel products experience during industrial processin
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