Effect of the Initial Microstructure and Thermal Path on the Final Microstructure and Bendability of a High Strength Ferrite-martensite Dual Phase Steel

Abstract

The combined effects of the initial microstructure and heating rate on ferrite recrystallization, austenite formation and transformation kinetics of a regular ferrite-martensite dual phase steel are investigated. Special attention is given to the effect of the martensite distribution on mechanical properties, particularly the bendability. Ferrite recrystallization in the cold rolled bainitic grade proceeds faster than in the ferrite-pearlite grade. The reason for this is the higher defect density leading to a lower activation energy. The junctions between the cementite and the ferrite grain boundaries are the preferred austenite nucleation sites. Those ferrite boundaries far away from the cementite can only be invaded by austenite through carbon supplied by cementite dissolution and carbon segregation. When using a high heating rate, nucleation of austenite can occur in a massive-like manner, and the austenite transformation kinetics is remarkably accelerated. The bendability of DP steel is determined by a combination of structural parameters, such as grain size, volume fraction as well as spatial distribution of martensite. A short in-line holding-step during heating can alleviate the banding severity and improve the bendability of this ferrite-martensite DP steel.

Unlocking the Mechanisms behind Austenite Formation of Cold‐Rolled Ferrite–Martensite Dual‐Phase Steels: The Role of Ferrite Recrystallization

“…These microstructural factors are dependent on the chemical composition and the processing parameters in the intercritical annealing, and on the initial hot-rolled microstructure due to the microstructural heritage. [1][2][3][4][5][6][7][8] Among these, austenite formation has been extensively studied, including the influence of initial microstructures, [9][10][11][12] heating parameters, [2,6,13] the underlying mechanisms such as elements partitioning, and interface characteristics accompanied by the α/γ interface movement. [14][15][16][17] It is known that ferrite recrystallization also occurs during the intercritical annealing process, and these two metallurgical phenomena usually have an interaction effect [18,19] especially under a relatively high heating rate.…”

“…These microstructural factors are dependent on the chemical composition and the processing parameters in the intercritical annealing, and on the initial hot‐rolled microstructure due to the microstructural heritage. [ 1–8 ] Among these, austenite formation has been extensively studied, including the influence of initial microstructures, [ 9–12 ] heating parameters, [ 2,6,13 ] the underlying mechanisms such as elements partitioning, and interface characteristics accompanied by the α / γ interface movement. [ 14–17 ]…”