Abstract
This study investigated the nucleation and growth mechanism of reaction layers and phases of hot-dipped boron steel in pure Al at 690 °C for 0–120 s. In the case of a dipping time of 30 s, reaction nuclei of width 10–15 μm and height 10 μm were formed on the steel surface in the flow direction of the liquid Al. This reaction layer was formed as a mixture of θ (Fe4Al13) phase of several nm to 2 μm, θ and η (Fe2Al5) of several nm, a columnar η region, and a β (FeAl) region of 500 nm thickness at the steel interface. At the grain boundaries of ferrite, in contact with the η phase, κ (Fe3AlC) was formed. Using the calculated Fe-Al phase diagram, it was determined that when Fe was dissolved in liquid Al from the steel above 2.5 at% (0.6 wt%), the θ phase was formed. Although most of the θ phases continuously grew toward the liquid phase, the θ phase in contact with the steel was transformed into the η phase with minimal differences in composition due to the inter-diffusion of Al and Fe. It was therefore concluded that the η phase formed at the interface became a growth nucleus and grew in a columnar form toward the steel.
Highlights
Boron steel, which has 5 ppm contents to give about GPa level tensile strength, has been widely used for ultra-high-strength steel (UHSS) for automobiles
The nucleation and growth of the reaction layer and phase formed on boron steel hot dipped in a pure liquid Al bath at 690 ◦ C for varying times was successfully investigated, which yielded the following observations and conclusions
When the dipping process was maintained for 30 s, the Al reacted with the iron oxide layer (Fe3 O4 ) on the surface of the steel and decomposed into Al2 O3
Summary
Boron steel, which has 5 ppm contents to give about GPa level tensile strength, has been widely used for ultra-high-strength steel (UHSS) for automobiles. The η phase grows toward the solid steel in the form of columnar grains and is known to be tongue- or sawtooth-shaped at the η phase/steel interface [7,8,9,10,11,12,13,14,15,16] The mechanism of this morphology is due to the high vacancy concentration in the c-axis of the crystal structure (orthorhombic, oC24) of Metals 2018, 8, 820; doi:10.3390/met8100820 www.mdpi.com/journal/metals. The nucleation reaction layer with a fine size (width 20 μm and height 1520 μm) at the initial stage of the steel dipping process was investigated in terms of type, size, distribution, and location to identify the nucleation mechanism and the growth process of the reaction layer and reaction phase on the surface
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