Abstract
The constituents, distribution, and characteristics of the phases formed on the coating layer of boron steel hot-dipped in Al-7wt%Ni-6wt%Si were evaluated in detail. In particular, the microstructure and phase constitution of the reaction layer were characterized. Moreover, the microstructural evolution mechanism of the phase was presented with reference to the (Al-7wt%Ni-6wt%Si)-xFe from the pseudo-binary phase diagram. The solidification layer consisted mainly of Al, Al3Ni, and Si phases. Reaction layers were formed in the order of Al9FeNi(Τ), Fe4Al13(θ), and Fe2Al5(η) from the solidification layer side. In addition, the κ (Fe3AlC) layer was formed at the Fe2Al5(η)/steel interface. From pseudo-binary phase diagram analysis, it was found that Fe4Al13(θ) can form when the Fe concentration is over 2.63 wt% in the 690 °C Al-7wt%Ni-6wt%Si molten metal. When the concentration of Fe increased to 10.0–29.0 wt%, isothermal solidification occurred in the Fe4Al13(θ) and Al9FeNi(Τ) phases simultaneously. Moreover, given that the T phase does not dissolve Si, it was discharged, and the Si phase was formed around the Al9FeNi(T) phase. The Fe2Al5(η) phase was formed by a diffusion reaction between Fe4Al13(θ) and steel, not a dissolution reaction. Moreover, Al2Fe3Si3(τ1) was formed at the Fe4Al13(θ)-Fe2Al5(η) interface by discharging Si from Fe4Al13(θ) without Si solubility. Furthermore, the Fe3AlC(κ) layer was formed by carbon accumulation that discharged in the Fe2Al5(η) region transformed from steel to Fe2Al5(η). The twin regions in the Fe4Al13(θ) and Fe2Al5(η) grain were due to the strains caused by the lattice transformation in the constrained state, wherein the phases are present between the Al9FeNi(Τ) layer and steel.
Highlights
In recent years, the automobile industry has been using boron steel, in which boron is added to a low alloy steel, for environmental friendliness, low fuel consumption, and low weight
The coating layer was classified into a solidification layer (SL) and reaction layer (RL)
Reaction layer (RL) is boron-steel/coating layer interface that composed of gray layers that observed some black particles inside and ivory phases that attached to gray layer and grown to solidification layer
Summary
The automobile industry has been using boron steel, in which boron is added to a low alloy steel, for environmental friendliness, low fuel consumption, and low weight. Boron steel is boron-added low alloy steel, and it is a type of advanced high-strength steel (AHSS) with a tensile strength of 1.5 GPa [1]. Due to the high strength of boron steel, cold forming is difficult, and the spring-back phenomenon occurs after forming. Hot stamping is performed to maintain the ductility by heating to a temperature of A3 or higher, press forming, and rapid cooling to maintain a high strength. Hot stamping involves heating at a temperature of A3 or higher, to ensure ductility, and quenching to maintain a high strength [2,3,4,5]
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