Influence of Mn Content on Fe-Zn Galvannealing Reaction of Si-added Steel Sheet

Abstract

Synopsis :The influence of the Mn content of Si-added steel sheets on the Fe-Zn galvannealing reaction was investigated. Three steel sheets, 1.5 mass%-Si-1.4, 1.9 and 2.7 mass%Mn, were annealed in a 10vol%H 2 -90vol%N 2 atmosphere. Si and Mn oxides were analyzed by reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and energy dispersive X-ray spectrometry. SiO 2 and Mn 2 SiO 4 formed as selective oxides at the steel surface after recrystallization annealing. The ratio of the oxide species changed depending on the Mn content in the steel. When the Mn content was lower, formation of SiO 2 was promoted and that of Mn 2 SiO 4 was suppressed. In the selective oxide layer which formed on the surface of the 1.5 mass%Si-1.4 mass%Mn steel sheets, Mn 2 SiO 4 formed at the outer side, and SiO 2 formed at the inner side. This can be explained by consideration of the thermodynamic oxygen potential gradient. Furthermore, areas where SiO 2 mainly formed and those where Mn 2 SiO 4 mainly formed were distributed on the surface of the 1.5 mass%Si-1.4 mass%Mn steel sheets. In this case, the Fe-Zn intermetallic compound (IMC) formed preferentially on the Mn 2 SiO 4 between the zinc coating and the substrate steel after galvanizing, and the Fe-Zn galvannealing reaction was suppressed on the SiO 2 layer. It is considered that a dense and continuous protective SiO 2 layer acted as a barrier to the Fe-Zn galvannealing reaction.

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.…”

Influence of Si Addition on Reduction Behavior of Fe Oxide on Mn-Added Steel Sheet

“…Although the chemical compositions of the internal oxides were not identified in this experiment, based on past studies [14][15][16] investigating the internal oxides of Si-and Mn-added steels, we assumed that the four internal oxides shown below were produced by the following equations. The equilibrium O 2 partial pressure of each reaction was calculated from the free energy change ΔG°1 7,18) of the respective reactions.…”