Relationship between the Morphology of Mn-oxides Simulated by Ion Plating and Phosphatability for Mn-added High Strength Cold-Rolled Steel Sheets

Abstract

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.