Study of the Effect of Thermomechanical Processing on Grain Refinement in HSLA Steels

The present study highlights the approach to multiscale analysis of the grain refinement during thermomechanical processing of microalloyed steels. HSLA steels for pipe-line use are introduced in which strain-induced precipitation and strain-induced transformations are controlled to produce ultra-fine grained microstructures. Multiscale modeling combining Digital Material Representation and Crystal Plasticity enables to gain a better fundamental understanding of mechanical response and microstructure evolution of precipitation strengthened austenite and ferrite of three microalloyed steel grades. The opportunities for the development of new thermomechanical processing schedules, focused on the grain refinement, are enhanced and the proposed models identifications procedure on the basis of the torsion tests at various forming conditions are presented and discussed. In the pipe-line grade steels, the combination of microalloying elements, accelerated cooling and transformation temperature, has led to much higher grain refinements and increased amount of bainitic ferrite microstructures than in the standard thermomechanical processing. Sensitivity analysis of studied microalloyed steel grades in the light of the discussed problems using microstructural analysis of obtained microstructures was also performed. Finally, the main challenges related to the multiscale modeling of proposed ideas are addressed.