Microstructural Evolution of Ultra Fine Grained C-Mn Steel Warm Rolling and Intercritical Annea

Abstract

The attainment of finer ferrite grain structure in low carbon is object of interest because significantly higher yield strengths and lower ductile-brittle transition temperatures can be predicted at the same time with ultra-fine grain sizes. This work verified the microstructural evolution of a low carbon 0.15%C-1.39%Mn steel after an ice brine quenching from 1200 °C followed by warm rolling and intercritical annealing at 800 °C at different times. To compare the final microstructure, a first group of specimens were heated at temperatures in the range from 660 °C to 800 °C during 30 min and quenching in ice brine. After quenching from 1200 °C, a second group of specimens were laboratory warm rolled at 700 °C and annealed at 800 °C, for 1, 60, 120 and 180 min, following air cooling or quenching. The final microstructure of all specimens was analyzed by quantitative metallography using optical and scanning electron microscopy. The initial steel grain size condition was 120 µm. The specimens, after the whole processing cycle achieved a microstructure with ferrite grain size between 1 µm and 1.5 µm. The ferrite grain size values changed until 50% for samples warm rolled and heat treated between the first and last annealing time.

Keywords

• ultra fine grain steel
• warm rolling
• intercritical annealing