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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.

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The thermo-mechanical control processing of Nb-Ti micro-alloyed steel by induction heating in the endless strip production (ESP) line was analyzed to better understand the microstructural evolution and Nb precipitation and dissolution behavior in austenite during rapid heating to high temperatures. The Nb-Ti micro-alloyed steel consisting of 0.05 wt% C and 0.05 wt% Nb was processed through simulated rough rolling at 1050 °C followed by rapid isothermal reheating at 1150 °C. The austenite coarsening behavior and the Nb dissolution behavior at different holding times were compared, and the coarsening kinetics of austenite grains and the dissolution kinetics of precipitates were investigated. It was found that during induction heating, the size of austenite grains gradually increased with the isothermal time, and the amounts of precipitates were greatly reduced. Round precipitates of (Ti, Nb) (C, N) and square precipitates of Ti (C, N) gradually dissolved into the austenite matrix with the holding time. The Nb content in the solution increased from 0.0137 to 0.0299 wt% as the holding time increased from 1 to 40 s; therefore, about 59.8% of the total Nb content dissolved into the austenite matrix during the induction heating process.

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