Enhancement of Properties of Micro-alloyed Low-Carbon Ni-Added Steel by Thermomechanical Treatment
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JMEPEG (2020) 29:7952–7963 https://doi.org/10.1007/s11665-020-05311-w
Enhancement of Properties of Micro-alloyed Low-Carbon Ni-Added Steel by Thermomechanical Treatment D. Roy, A.K. Gupta, Md.S. Alam, S. Srikanth, and B.K. Jha Submitted: 4 June 2020 / Revised: 30 October 2020 / Accepted: 1 November 2020 / Published online: 16 November 2020 The concurrent use of micro-alloying additions and thermomechanical treatment is a common practice for obtaining high-quality grades of forged steels. Judicious use of chemistry, heat treatment schedule, and controlled processing can lead to a wide range of microstructure and thus final properties suitable for the production of different parts for the automobile industry. In this paper, thermomechanical treatment of Niadded micro-alloyed steel containing Nb, V, Ti has been carried out with different deformation schedules (50-70%) at high temperatures followed by the varying cooling rate (i.e., forced air cooling, oil quenching, and water quenching). The cooling rates for three conditions were measured as 5 °C/s for forced air cooling, 40 °C/s for oil quenched, and 110 °C/s for water quenched samples. Thermomechanical and posttempering treatment cycles were employed for the steel with 1150 and 900 °C as upper and lower forging temperatures. The best combination of properties viz; YS: 1044 MPa, UTS: 1308 MPa, % El: 16.7, %RA: 57 and Impact Toughness: 50 J/cm2 could be achieved for 70% deformed samples, followed by forced air cooling. The reasons have been attributed to the segregation of Ti & Nb carbides at grain boundaries as revealed by microstructural analysis. This has led to restriction of grain boundary coarsening at high temperatures. Further, fine vanadium carbide and the presence of retained austenite have contributed to improving the elongation and toughness values. Keywords
Charpy impact, EDS, forging, heat treatment, micro-alloyed steels, microstructure, Nb & V carbides, retained austenite, tensile properties, Ti
1. Introduction The continual demand for better structural steels has resulted into innovative efforts leading to the development of highstrength low-alloy (HSLA) or micro-alloyed steels (Ref 1-3). Micro-alloyed steels are complex material, which requires highly controlled processing to the desired high strength and toughness through a combination of small grain size and distribution of fine precipitates. Micro-alloying with Nb, V, and Ti either singly or in combination leads to fine grain size, while in solution in austenite. On subsequent cooling the precipitates as carbide and carbonitrides which are fine enough to offer hindrance to mobile dislocation leading to a significant increase in strength. Three possible thermal regimes of hot working exist for the micro-alloyed steels, namely recrystallization, partial recrystallization, and no recrystallization. Controlled rolling therefore aimed at austenite conditioning (i.e., austenite grain refinement) through repeated recrystallization in the roughing section of rolling mills with thermal regime hot workin
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