(Nb,Ti)(C,N)‐Precipitates in High‐Strength Low‐Alloyed Steels—Modeling, Characterization and Validation

Abstract

The chemical composition and microstructural evolution during thermomechanical treatments of high-strength low-alloy steels containing Nb and Ti determine the mechanical properties. To optimize processing and heat treatment, it is important to characterize the microstructures and to understand how the mechanical properties are affected by the different elements of the microstructures. Herein, it has been attempted to investigate the nucleation and growth of niobium–titanium carbonitrides using transmission electron microscopy (TEM)-energy-dispersive X-ray spectroscopy measurements. Since TEM measurements alone tend to suffer from poor statistics, an attempt is made to establish a kinetic/thermodynamic model that is representative for larger volumes of material while giving fair credit to the local TEM results. The present work goes beyond the simulation of temperature-dependent average compositional changes of precipitates. Instead, this study aims to capture the entire compositional spectrum of precipitates and their evolution during processing and heat treatment. The model used is based on the CALPHAD approach but has been extended by a Monte Carlo approach. The focus is on the rationalization of particle compositions that are experimentally observable by TEM and cannot be obtained by conventional mean-field models based on extremum principles.