Effects of the Surface State on Phosphatability for High Strength Cold-Rolled Steel Sheets

Abstract

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.

Relationship between Morphology of Mn Oxides Simulated by Ion Plating and Phosphatability of Mn-added High-strength Cold-rolled Steel Sheets

“…The aforementioned results demonstrate the importance of controlling the morphology of monoxides to obtain small particles, in addition to the previously reported method of controlling the species and amount of the respective oxides, 19,20) for achieving good phosphatability of Mn-based high-strength cold-rolled steel sheets. Fig.…”

“…The thickness of the target oxide film was 50 nm, which is similar to the thickness of the oxide film formed on real high-strength steel sheets. 19,20) Next, the sample was heated to 900°C in a 5 vol% H 2 -95 vol% N 2 atmosphere at a rate of 10°C/s. Annealing was then conducted by maintaining the sample at 900°C for 120 s. After the annealing process, the sample was cooled to room temperature under nitrogen gas.…”

“…These results show that the thickness of the oxide layer produced by the ion-plating method was approximately 50 nm for both the Mn-O and Mn-B-O films. Thus, the amount of oxides added to the Mn-O and Mn-B-O films was equal to that present on high-tensile steel 19,20). 3.2.…”