Ab Initio Calculations on Elastic Properties of IF Steel Matrix Phase at High Temperature Based on Lattice Expansion Theory

Abstract

Elucidating the evolution law of the elastic properties of the matrix phase is of great significance for the control of steel properties and quality during continuous casting and subsequent heat treatment. In this paper, thermal expansion experiments and ab initio calculations are used to study the elastic properties of the interstitial free (IF) steel matrix phase in different magnetic states and crystal structures. The results show that the bulk modulus B and the tetragonal shear elastic constant C’ for the entire temperature range decrease with increasing temperature, but C44 is the opposite. While from paramagnetic (PM) to ferromagnetic (FM) state, C’(C44) have changed ~188% (~27%), B increases by ~55% during the crystal structure change (fcc→bcc). With the FM to PM state, the Zener anisotropy parameter increases sharply, and Young’s modulus decreases significantly in the [001] direction; the maximum difference is ~76 GPa. The evolution rate of average Young’s modulus in single bcc-phase FM (fcc-phase PM) range reaches ~5.5(~5.6) × 10−2 GPa K−1. The research provides an effective method for ab initio calculation of the elastic properties of interstitial free and ultra-low carbon steels at high temperature, also furnishing a basis for the application of ab initio calculations to the high temperature performance of steel materials.

Effect Mechanism of α-Ferrite Sustained Precipitation on High-Temperature Properties in Continuous Casting for Peritectic Steel

“…The tensile and yield strength, whichever w they were obtained, show a fairly similar pattern of evolution. In the high-temperatu austenite phase region and the initial stage of α-ferrite precipitation (i.e., 750~900 °C), t high-temperature strength is not sensitive to temperature fluctuation, primarily due to t combined effect of the crystal structure and the transition of the magnetic state [33]. T tensile strength and yield strength start to improve significantly below 750 °C.…”

“…The tensile and yield strength, whichever way they were obtained, show a fairly similar pattern of evolution. In the high-temperature austenite phase region and the initial stage of α-ferrite precipitation (i.e., 750~900 • C), the high-temperature strength is not sensitive to temperature fluctuation, primarily due to the combined effect of the crystal structure and the transition of the magnetic state [33] With the pursuit of high-quality steel, the re-improvement of mechanical prope while avoiding cracks in the casting slab has become a further ambition. The developm of tensile and yield strengths during the austenite phase transition is given in Figur The tensile and yield strength obtained from the engineering stress-strain curve is den as σb,e and σs,e, respectively.…”

“…Overall, these studies primarily focus on the effect of the formation and distribution of film-like ferrite on hot ductility in the pre-austenite phase transition stage, with fewer quantitative descriptions of the properties evolution during the whole α-ferrite precipitation advancement process. Actually, as the austenite phase transition process advances, the microstructure of the as-cast slab undergoes complex changes such as the generation of new phases, the depletion of the original phases, and the change in the crystal structure of the matrix phases [30,31], which has a substantial impact on the properties [32,33]. Furthermore, avoiding cracks is only a basic requirement for high-quality steel products, and further improvements in strength and toughness should be considered as a new aim.…”