The effect of heating rate and soaking time on microstructure of an advanced high strength steel

TL;DR

This study investigates how heating rate and soaking time influence the microstructure of an advanced high strength steel during ultrafast heat treatment, revealing finer microstructures and phase transformation dynamics.

Contribution

It provides detailed microstructural analysis of ultrafast heat treatment effects on low carbon steel, highlighting the impact of soaking time on phase fractions and grain structure.

Findings

Ultrafast heating produces finer microstructures than conventional heating.

Soaking time significantly affects martensite volume fraction in ultrafast heated samples.

Recrystallization and phase transformations are influenced by heating rate and soaking duration.

Abstract

This work focuses on the effect of soaking time on the microstructure during ultrafast heat treatment of a 50% cold rolled low carbon steel with initial ferritic-pearlitic microstructure. Dilatometry analysis was used to estimate the effect of heating rate on the phase transformation temperatures and to select an appropriate inter-critical temperature for final heat treatments. A thorough qualitative and quantitative microstructural characterization of the heat treated samples is performed using a wide range of characterization techniques. A complex multiphase, hierarchical microstructure consisting of ferritic matrix with embedded martensite and retained austenite is formed after all applied heat treatments. In turn, the ferritic matrix contains recrystallized and non-recrystallized grains. It is demonstrated that the ultrafast heating generally results in finer microstructure compared to the conventional heating independently on the soaking time. There is a significant effect of the soaking time on the volume fraction of martensite of the ultrafast heated material, while in the samples heated with conventional heating rate it remains relatively unchanged during soaking. Recrystallization, recovery and phase transformations occurring during soaking are discussed with respect to the applied heating rate.