Effect of alloying and microstructure on formability of advanced high-strength steels processed via quenching and partitioning

TL;DR

This study investigates how alloying and microstructure influence the formability of advanced high-strength steels processed via quenching and partitioning, revealing complex relationships between microstructure, retained austenite, and formability.

Contribution

It provides new insights into the effects of microstructure and alloying on formability and the transformation behavior of Q&P steels under different stress states.

Findings

Higher retained austenite increases ductility but may reduce formability due to microcrack formation.

Multiaxial stress enhances transformation of retained austenite, improving plastic strain accumulation.

Tempered martensitic matrix influences crack initiation and deformation capacity.

Abstract

The article focuses on the effect of alloying and microstructure on formability of advanced high strength steels (AHSSs) processed via quenching and partitioning (Q&P). Three different Q&P steels with different combination of alloying elements and volume fraction of retained austenite are subjected to uniaxial tensile and Nakajima testing. Tensile mechanical properties are determined, and the forming limit diagrams (FLDs) are plotted. Microstructure of the tested samples is analyzed, and dramatic reduction of retained austenite fraction is detected. It is demonstrated that all steels are able to accumulate much higher amount of plastic strain when tested using Nakajima method. The observed phenomenon is related to the multiaxial stress state and strain gradients through the sheet thickness resulting in a fast transformation of retained austenite, as well as the ability of the tempered martensitic matrix to accumulate plastic strain. Surprisingly, a Q&P steel with the highest volume fraction of retained austenite and highest tensile ductility shows the lowest formability among studied grades. The latter observation is related to the highest sum of fractions of initial fresh martensite and stress/strain induced martensite promoting formation of microcracks. Their role and ability of tempered martensitic matrix to accumulate plastic deformation during forming of Q&P steels is discussed.