Effect of Deformation Sequence and Coiling Conditions on Precipitation Strengthening in High Ti–Nb-Microalloyed Steels

Abstract

In this work, multipass torsion tests followed by coiling simulations under different conditions have been performed with a reference Nb (0.03 pct) and a high Ti (0.1 pct)–Nb-microalloyed (0.03 pct) steel. In the case of the high Ti steel, estimated yield strengths close to or over 700 MPa were obtained for some of the conditions researched. However, a very significant effect of previous austenite grain size and strain accumulation on precipitation strengthening has also been observed. As a result, depending on deformation sequence and final cooling conditions, the coiling simulation temperatures that lead to the highest mechanical strength varied from 600 °C to 500 °C. The effect of increasing strain accumulation was mainly related to higher phase transformation temperatures, which led to a lower driving force for precipitation and higher microalloying element diffusivity, resulting in the formation of less and coarser precipitates.

Effect of Coiling Temperature on Microstructure and Properties of a Ti–Nb Microalloyed High‐Speed Guardrail Steel

“…[38] As the coiling temperature increases from 570 to 620 °C, the kinetic diffusivity of Ti and Nb increases, which corresponds to the fact that although the equilibrium solid solubility of carbides decreases slightly with the increase of coiling temperature, the diffusion coefficients of controlling elements such as Ti, Nb, and C are relatively large, and the precipitation nucleation rate is faster, which accelerates the precipitation nucleation of (Ti, Nb)C, resulting in an increase in the number density and a decrease in the size of precipitates during coiling at 620 °C. [36,39] In contrast, the average size of (Ti, Nb)C particles increases from 6.39 nm to 7.11 nm with the increase of the coiling temperature from 620 to 670 °C because the size of precipitates is closely related to the Ostwald ripening rate. The change of ripening rate of precipitates in ferrite with coiling temperature is shown in Figure 14.…”

Production of a Non-Stoichiometric Nb-Ti HSLA Steel by Thermomechanical Processing on a Steckel Mill

“…Recently, special attention has been paid to the significant precipitation hardening obtained by addition of high Ti levels (higher than 0.05 wt%) in low-carbon steels, due to the formation of a high density of ultrafine precipitates during or after phase transformation [9,11]. Commonly, high Ti is added in combination with other microalloying elements, such as Mo [12,13], V [14], or Nb [15][16][17]. Among the different alloy concepts, the most investigated one is Ti-Mo steel grade.…”