Processing-Microstructure-Property Correlation for Producing Stretch-Flangeable Grade Dual-Phase Steel

Abstract

Continuous annealing simulation experiment of a large sample (220 mm length and 110 mm width) is carried out for a better understanding of the role of annealing parameters on microstructure and mechanical properties so as to develop dual-phase steel with consistent quality. A typical complete annealing cycle used in industry for producing dual-phase structure was divided into several thermal stages to investigate various mechanisms at different thermal stages. A correlation among microstructure, process parameters and mechanical property is established by varying the important annealing process parameters. Based on the systematic study at each thermal stage, the annealing parameters that have greater influence on mechanical properties were identified. A modified complete thermal cycle, after control of these key parameters, produced a better distribution of fine martensite along the ferrite grain boundaries and thus enhanced mechanical properties. The study showed that higher amount and uniform distribution of fine martensite with granular morphology along the ferrite grain boundaries for a fixed composition of steel is essential for improved mechanical properties. The uniform distribution of fine martensite with required amount within the ferrite matrix not only enhanced strength–ductility balance but also improved stretch-flangeability.