Effects of Coiling Temperature on Microstructure and Precipitation Behavior in Nb–Ti Microalloyed Steels

Abstract

This paper presents our latest studies on the effects of coiling temperature on the microstructure, precipitation behavior, and mechanical properties of an Nb-Ti microalloyed steel produced by endless strip processing (ESP) and coiled at different temperatures. The amounts of soluble elements were measured using inductively coupled plasma optical emission spectrometry (ICP-OES). The microstructure and precipitates were analyzed using SEM, EBSD, TEM, and electrolytic dissolution and filtration tests. The results revealed that large amounts of microalloying elements were still in solution before coiling. As the coiling temperature decreased from 600°C to 560°C, the content of acicular ferrites (AF) increased and the average ferritic grain size was refined from 2.01 μm to 1.29 μm, the yield strength and tensile strength of the tested steel increased by 22 MPa and 20 MPa, respectively, under the effect of microstructural strengthening. As the coiling temperature increased from 600°C to 640°C, the mass fraction of precipitates increased from 0.083% to 0.110% and the percentage of fine precipitates (smaller than 18 nm) increased from 12.2% to 14.7%; the intense precipitation strengthening effect increased the yield strength and tensile strength by 35 MPa and 42 MPa, respectively. Therefore, as the coiling temperature decreased from 640°C to 560°C, the strength of the tested steel decreased first and then increased while the elongation decreased steadily from 18.9% to 14.1% due to the increasing content of AF.

Effect of V on the Precipitation Behavior of Ti−Mo Microalloyed High-Strength Steel

“…The second type (P2) is verified as cementite mainly distributed in the grain boundaries. According to the previous studies [ 18 , 19 ], these (Ti, Mo, V)C particles are mainly precipitated during the soaking at high temperature.…”

“…The supersaturated solute microalloying elements will further precipitate in the ferrite during the coiling process. Compared to austenite, the precipitation precipitated in ferrite is smaller in size, thereby producing stronger precipitation strengthening [ 18 , 19 , 26 ]. It should be noted that the contents of solid solution of microalloying elements and carbon in the austenite at 860 °C are used as the initial content in the ferrite calculation model.…”

Effect of Microalloyed Elements M (M = Ce, Ti, V, and Nb) on Mechanical Properties and Electronic Structures of γ‐Fe: Insights from a First‐Principles Study

“…At present, industrialized developed countries regard microalloyed steel as the most important research and development direction in the iron and steel industry. [1][2][3][4][5] Previous studies have shown that adding Ti, V, and Nb elements separately or jointly can refine the structure and enhance precipitation strengthening. [6,7] Based on the Ashby-Orowan equation, [8] the size of the precipitation significantly affects the precipitation strengthening.…”

Effect of coiling temperature on impact toughness of hot rolled ultra-high-strength multiphase steel strips

“…Regarding thermomechanically processed and controlled cooled high-strength strip steels, the role of processing parameters such as soaking temperature, start-and finish-rolling temperatures, rolling reduction ratio, cooling start-and end temperatures and cooling rate on the microstructure and mechanical properties have been studied extensively [4][5][6]. It is noteworthy that UHSS strips (< 10 mm thickness) are usually coiled after finish rolling, and use accelerated cooling to a lower temperature range (typically, 350 -460 C) to achieve high strength by the formation of bainite-martensite mixed microstructures [7][8][9][10][11][12][13]. An increase in strength, however, is often accompanied by a decrease in ductility and impact toughness [14].…”