Strain-induced precipitation in Ti microalloyed steel by two-stage controlled rolling process

Abstract

Strain-induced precipitation kinetics and precipitates' characteristics in a Ti microalloyed steel subjected to a two-stage controlled rolling process, simulating rough and finish rolling process of industrial production, were quantitatively investigated by stress relaxation experiments and transmission electron microscopy (TEM), as well as inductively coupled plasma-atomic emission spectroscopy (ICP-AES). In the present work, precipitation during the cooling stage after rough rolling was taken into consideration for the first time. It was found that a small amount of Ti-bearing precipitates, consuming about 10% of the elemental Ti, were formed on dislocations after rough rolling and cooling to 900 °C. The obtained precipitation-time-temperature (PTT) curves exhibited a classic C shape with a nose temperature of 900 °C and the shortest incubation time of 60 s. The PTT curves moved to the lower left by the introduction of 20% deformation at 1050 °C, which was attributed to more nucleation sites for strain-induced precipitation and the decrease of Ti concentration in deformed austenite. During the stress relaxation stage, strain-induced precipitates preferentially nucleated on dislocations and sub-boundaries and were identified as TiC particles. The mean size of precipitated TiC particles increased from 10.2 ± 2.1 to 25.2 ± 2.8 nm, as the holding time was increased from 100 to 1800 s at 900 °C. When the holding time exceeded 600 s the migration of austenite grain boundaries could not be completely inhibited due to the coarsening of precipitates. Isothermal treatment at 900 °C with a holding time of 60–100 s is suggested here as a viable combination for the processing of the 0.05C-0.21Si-1.05Mn-0.13Ti (wt. %) steel due to the shortest incubation time period and effective pinning of grain boundaries by strain-induced precipitates.