Effects of pre-annealing conditions on the microstructure and properties of vanadium-bearing dual-phase steels produced using continuous galvanizing line simulations

Abstract

It is well known that the fracture behavior and weldability of steel often vary inversely with the carbon content. In the case of thin sheet intended for automotive applications, this means spot weldability and tensile ductility. However, the question arises, what is the reasonable maximum strength attainable in a low-carbon dual-phase (DP) steel under the constraints of: (1) with carbon levels at 0.1 wt%, (2) with the product (UTS × % TE) > 18,000 MPa, and (3) with processing on a simulated continuous hot-dipped galvanizing line. This study has focused on experimental compositions intended to meet DP steel grade properties in excess of 780 MPa UTS. This kind of advanced high-strength steel (AHSS) is an integral part of mass reduction programs for the body-in-white for the automotive industry. The family of steels investigated here is based on a low-carbon, aluminum-killed base steel containing Cr and Mo. In this study, the effects of the presence of V were studied. In addition, there were three processing variables investigated in terms of their respective effects on microstructure and mechanical properties: first, hot band coiling temperature; second, % cold reduction; and third, the thermal path through the CGL process. The thermal paths explored included both a standard galvanizing process and a new supercooling process; these were studied using a CGL simulation. This research revealed that all three variables can have a significant effect on the final microstructure and properties.