This study investigated the effects of surface conditions on phosphatability for Si-and Mn-containing high-strength cold-rolled steel sheets. Si-, Mn-, and Fe-containing oxides of approximately 3 μm form on the steel surface after annealing. During 3 s pickling using nitric acid, these oxides are partially removed, exposing the steel surface. Additional pickling （6 s pickling time） mostly removes Si-, Mn-, and Fe-containing oxides, but Fe-based oxides of 50 nm maximum thickness are formed on the steel surface. These observations suggest that Si-, Mn-and Fe-containing oxides （about 3 μm thickness） and Fe-based oxides （about 50 nm thickness） interfere with the steel dissolution in the phosphate solution. Results demonstrate the importance of controlling the Fe-based oxide formation as well as that of Si-and Mn-containing oxides to obtain good phosphatability.

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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.

It is well known that Si, Mn and B, the alloying elements for high strength steel sheets, easily form oxides on the steel surface during annealing in a reducing atmosphere, and those oxides have a large influence on the surface performance of steel sheets, such as phosphatability. In this work, we discovered that the oxidation behavior of Mn-added high strength cold-rolled steel sheets could be simulated on mild steel sheets by using an ion plating method and investigated the relationship between the morphology of Mn oxides and phosphatability under the condition that both the amount and kind of Mn oxides were fixed. In a simulated Mn-O layer, fine surface oxides, which covered most of the steel surface, were observed after annealing. On the other hand, in a Mn-B-O layer, large globular surface oxides were observed on the steel surface, and the Fe surface was partially bare. The B-Mn compound oxide is considered to be in a molten phase during annealing because the melting point of the compound oxide is lower than the annealing temperature, and as a result, it is thought that large B-Mn compound oxides coagulate and grow during annealing. In addition, it was found that the large B-Mn compound oxides (about 500 nm) interfere with steel dissolution in the phosphate solution. These results demonstrate the importance of controlling the morphology as well as the amount and kind of surface oxides for obtaining good phosphatability of Mn-added high strength cold-rolled steel sheets.