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

Abstract

Obtaining high levels of mechanical properties in steels is directly linked to the use of special mechanical forming processes and the addition of alloying elements during their manufacture. This work presents a study of a hot-rolled steel strip produced to achieve a yield strength above 600 MPa, using a niobium microalloyed HSLA steel with non-stoichiometric titanium (titanium/nitrogen ratio above 3.42), and rolled on a Steckel mill. A major challenge imposed by rolling on a Steckel mill is that the process is reversible, resulting in long interpass times, which facilitates recrystallization and grain growth kinetics. Rolling parameters whose aim was to obtain the maximum degree of microstructural refinement were determined by considering microstructural evolution simulations performed in MicroSim-SM® software and studying the alloy through physical simulations to obtain critical temperatures and determine the CCT diagram. Four ranges of coiling temperatures (525–550 °C/550–600 °C/600–650 ° C/650–700 °C) were applied to evaluate their impact on microstructure, precipitation hardening, and mechanical properties, with the results showing a very refined microstructure, with the highest yield strength observed at coiling temperatures of 600–650 °C. This scenario is explained by the maximum precipitation of titanium carbide observed at this temperature, leading to a greater contribution of precipitation hardening provided by the presence of a large volume of small-sized precipitates. This paper shows that the combination of optimized industrial parameters based on metallurgical mechanisms and advanced modeling techniques opens up new possibilities for a robust production of high-strength steels using a Steckel mill. The microstructural base for a stable production of high-strength hot-rolled products relies on a consistent grain size refinement provided mainly by the effect of Nb together with appropriate rolling parameters, and the fine precipitation of TiC during cooling provides the additional increase to reach the requested yield strength values.

Cooling pattern on the run-out table of a hot rolling mill for an HSLA steel: a finite element analysis

“…The density, specific heat, thermal conductivity coefficient, and emissivity of HSLA steel were considered to be temperature-dependent [30], Table 1. The phase transformation latent heat was neglected in the present research.…”

Study on Austenite Transformation and Growth Evolution of HSLA Steel

“…In addition, the austenite grain growth needed not only driving force but also the mobility of the grain boundary. The precipitates in the sample could hinder the movement of the grain boundary, reducing its mobility, which also led to a smaller austenite grain size when the heating temperature was lower [12,22]. With the increase in the heating temperature, the driving force of austenite growth increased, and some precipitate began to dissolve into austenite, reducing the barrier of the grain boundary movement so that the austenite grain growth rate increased.…”

“…When the heating temperature was low, the formed austenite grains were even and fine, which helped to reduce the internal stress and refine the structure in the subsequent deformation and transformation process, but the alloy elements could not be fully dissolved, which easily caused structural and composition segregation. Moreover, the carbonitrides in HSLA steel affect the austenite grain growth; for example, Ti-rich rectangular carbonitrides have high thermal stability and can be pinned to austenitic grain boundaries, and the dissolution of Nb in austenite can lead to the rapid growth of austenite grains, which slows down ferrite transformation and is beneficial to the formation of bainite [12][13][14]. Nevertheless, the pinning of precipitates in the segregation band of the microstructure can lead to the abnormal grain growth of austenite during the reheating process [15].…”

Effect of Coiling Temperature on Microstructures and Precipitates in High-Strength Low-Alloy Pipeline Steel after Heavy Reduction during a Six-Pass Rolling Thermo-Mechanical Controlled Process

“…For Nb-Ti HSLA pipeline steel, Nb and Ti play a significant role in achieving their strength [3,24], as the precipitation of Nb and Ti is very sensitive to CT [19,20]. For Nb-Ti micro-alloyed steel, the precipitates begin to form at a CT of 675 • C. The intermediate precipitates are small in size and randomly distributed in the matrix at a CT of 650 • C [20][21][22][23][24][25]. During the coiling process in TMCP, large (Nb, Ti)C particles and small spherical (Nb, Ti)C particles form in Nb-Ti HSLA pipeline steel, which leads to dislocation pinning [21].…”