Design of Thermomechanical Processing Routes for Advanced High Strength Steels: Microstructure and Properties

Abstract

The steel industry has shown increasing interest in developing high-strength steels to meet the automotive industry's weight reduction goals for vehicle safety and emissions reduction targets. Medium manganese steels (MMn-AHSS) are promising candidates for third-generation advanced high-strength steels (AHSS) due to their superior mechanical properties and potentially lower cost compared to other AHSS. Different heat treatment processes have been developed to tailor the microstructure and mechanical properties of the MMn-AHSS. This study introduces a novel two-stage thermomechanical process with different cold rolling directions on a range of the MMn-AHSS. The research compares the effect of the rolling direction, initial dislocation density, initial martensitic microstructure, and alloying with aluminium. The base steel for the study was Fe-0.068C-11.3Mn (wt.%) and the Al-bearing grade was Fe-0.068C-11.3Mn-1.1Al (wt.%). Thermodynamic calculations and experiments were used to determine the transformation temperatures. The evolution of the microstructure and mechanical properties was studied using different techniques. The results revealed that Al increases the austenite start and finish temperatures, and decreases the martensite start temperature. The average grain size of the ferrite (α) and austenite (γ) phases, however, remained unchanged. Increasing the temperature and time of the intercritical annealing (IA) resulted in an increase in the γ volume fraction and larger γ and α grain sizes. Raising the dislocation density of the initial martensitic microstructure led to an 8% increase in the retained γ. The specimen with an initial deformed martensite microstructure after the IA showed the higher mechanical properties than the specimen with the initial quenched and deformed martensite microstructure after the IA. The findings demonstrated that thermomechanical processing with TD cold rolling yielded superior mechanical properties compared to RD cold rolling.