Tracing Microalloy Precipitation in Nb-Ti HSLA Steel during Austenite Conditioning

Abstract

The microalloying with niobium (Nb) and titanium (Ti) is standardly applied in low carbon steel high-strength low-alloy (HSLA) steels and enables austenite conditioning during thermo-mechanical controlled processing (TMCP), which results in pronounced grain refinement in the finished steel. In that respect, it is important to better understand the precipitation kinetics as well as the precipitation sequence in a typical Nb-Ti-microalloyed steel. Various characterization methods were utilized in this study for tracing microalloy precipitation after simulating different austenite TMCP conditions in a Gleeble thermo-mechanical simulator. Atom probe tomography (APT), scanning transmission electron microscopy in a focused ion beam equipped scanning electron microscope (STEM-on-FIB), and electrical resistivity measurements provided complementary information on the precipitation status and were correlated with each other. It was demonstrated that accurate electrical resistivity measurements of the bulk steel could monitor the general consumption of solute microalloys (Nb) during hot working and were further complemented by APT measurements of the steel matrix. Precipitates that had formed during cooling or isothermal holding could be distinguished from strain-induced precipitates by corroborating STEM measurements with APT results, because APT specifically allowed obtaining detailed information about the chemical composition of precipitates as well as the elemental distribution. The current paper highlights the complementarity of these methods and shows first results within the framework of a larger study on strain-induced precipitation.

Thermomechanical Processing of Steels

“…The present Special Issue on Thermomechanical Processing of Steels includes eleven research papers [1][2][3][4][5][6][7][8][9][10][11]. A wide range of steel grades are covered in these papers.…”

“…A wide range of steel grades are covered in these papers. Although most of the papers deal with low carbon microalloyed grades [1,3,6,9,10], several papers study ferritic stainless steels [2,5,7] and others focus on grades such as Fe-Al-Cr alloys [4], medium-Mn Nb microalloyed steels [8], and medium carbon V microalloyed grades [11].…”

Influence of Welding Parameters and Filler Material on the Mechanical Properties of HSLA Steel S960MC Welded Joints

“…The high strength and good formability of the assessed steel, S960MC, is also ensured by the fine-grained microstructure achieved by the thermo-mechanical controlled process. The presence of Nb, Ti, and V elements is particularly important in this process, as they form dispersive precipitates of the (Ti, Nb) (C, N) type, which provide precipitation hardening and sufficient grain refinement [1,5]. These elements' function mechanisms vary, and have different utilization ranges.…”

Influence of microalloying elements (Ti, Nb) and nitrogen concentrations on precipitation of pipeline steels—A thermodynamic approach

“…Precipitates formed in the first stage are very stable and generally too coarse to influence on austenite recrystallization. Segregation of titanium and nitrogen at the late stages of solidification consumes a substantial fraction of the added titanium to form coarse TiN precipitates; the remaining titanium, precipitates in austenite 23 . Fine titanium nitride precipitates extend the temperature range over which the grain boundary pinning is effective, due to a significantly higher solution temperature of these precipitates compared to other precipitates that are known to restrict grain growth 1,2 .…”

“…Therefore, extensive efforts have been made to optimize the Ti/N ratio as a major microstructural control factor for a variety of steels 4,5,24‐26 . Soluble Nb in austenite reduces the temperature at which Eutectoid transformation occurs and its reduced solubility with temperature in austenite results in formation of niobium carbides and carbonitrides at temperatures lower than precipitation temperature of TiN as a function of Nb concentration 23 . These fine precipitates increase the recrystallisation temperature during hot rolling and refining microstructure along with titanium carbides 13,23,27 …”