The Hot Ductility, Microstructures, Mechanical Properties and Corrosion Resistance in an Advanced Boron-Containing Complex Phase Steel Heat-Treated Using the Quenching and Partitioning (Q&P) Process

Abstract

The objective of this research work is to obtain the hot ductility behavior, and the structural, microstructural and mechanical characteristics of one of the latest generation of AHSS steels, a complex phase (CP) steel microalloyed with boron (0.006 wt.%), processed by hot and cold rolling operations and heat-treated using two different quenching and partitioning (Q&P) treatments, a one-step partitioning (quenching to 420 °C) and the other a two-step partitioning (quenching to 420 °C and reheated to 600 °C). The results show that boron has a marked effect on the solidification process of the CP steel, refining the austenitic grain size. Due to its refinement, the boron-containing steel had better ductility throughout the temperature range examined (700–900 °C), i.e., the hot ductility trough. Thus, the minimum percentage of reduction in area (%RA) value occurring at 800 °C was 43% for the boron-free steel, compared with 58% for the boron-containing steel. Hence, cracking would not be a problem when straightening the strand on continuous casting. The benefit of boron addition on the room temperature properties was found to be very marked for the higher temperature two-step partitioning treatment, giving a yield stress of 1200 MPa, a UTS (ultimate tensile strength) of 1590 MPa and a total elongation above 11%. The final Q&P microstructure, in both one- and two-step partitioning conditions, consisted of retained austenite (RA-γ), martensite and ferrite islands in a bainitic matrix. Furthermore, the boron treated steel on quenching produced a greater amount of RA-γ, which accounted for its better room temperature ductility and produced a martensitic matrix rather than a bainitic one, giving it greater strength. The addition of boron improved the corrosion resistance of this type of third generation AHSS steel.