Effects of Solid-Solute V on the Phosphatability of Hot-Rolled Steel Sheets

Abstract

The phosphatability of hot-rolled steel sheets has become increasingly important with application of higher strength and thinner steel sheets to automotive parts. Although V is an alloying element which is often added to high strength hot-rolled steel sheets, the effect of V on phosphatability was still unclear. This study investigated the phosphatability of V-added hot-rolled steel sheets by using V-free, 0.20% V and 0.47% V steel sheets as test specimens. After phosphate treatment, phosphate crystals covered the whole surface of the V-free and 0.20% V steel sheets, but there were no phosphate crystals on the surface of the 0.47% V steel sheets. In order to clarify the mechanism, potentiostatic electrolysis in the phosphate treatment solution was carried out. Phosphate crystals were found on the surface of the V-free steel sheet after both cathodic and anodic polarization. In contrast, no phosphate crystals were found on the surface of the 0.47% V steel sheet after anodic polarization, but similarly to the V-free steel sheet, phosphate crystals had formed after cathodic polarization. A surface analysis by XPS revealed that V oxides had precipitated on the surface of the 0.47% V steel sheet after anodic polarization. Evolution of hydrogen ions in the oxidation reaction of V inhibits the formation of phosphate crystals. Thus, oxidation of V on the steel surface in the phosphate treatment solution was identified as the factor responsible for deterioration of the phosphatability of V-added hot-rolled steel sheets.