12th International Conference on Zinc & Zinc Alloy Coated Steel Sheet - GALVATECH 2021

To improve the surface feature of high-Mn steel, such as wettability with liquid Zn, a functionally graded multilayer was fabricated on the top of as-cast high-Mn steel slab by laser cladding process. For the alloying material, low carbon steel powder (0.07C, 1.83Mn) was deposited. To investigate the optimum condition for wettability, four different samples were prepared: bare high-Mn steel sample (LC-0), once, twice and three times cladded samples (LC-1, 2, 3). The thickness of clad layers was 2~4mm and that of slab was 140mm. Due to substrate re-melting effect of laser cladding, compositional gradient along the deposition direction appeared, resulting in different microstructure by layer. The first layer with 9% Mn had a dual-phase microstructure (martensite + interdentiric austenite) followed by bainitic and fully ferrite layers along the deposition direction. All samples were hot and cold rolled, and hot-dip galvanized in molten zinc bath. Defects in Zn coating and oxides, which were formed during the annealing prior to hot dipping, were investigated using SEM Energy Dispersive Spectrometry and X-ray Photoelectron Spectroscopy. As the number of deposited layer increased from LC-0 to LC-3, Mn content of the top surface of the sample decreased and the fraction of ferrite increased. MnO was dominant in LC-0, but as the number of deposited layer increased, the fraction of MnO-SiO2 oxides and Al2O3 increased. It is because Mn/Si and Al decreased as the number of clad layer increased and the diffusivity of Si and Al in ferrite is much higher than that of Mn in austenite. Because the laser cladding process turned coarse grains of the substrate into finer grains, oxides in LC-1, 2, 3 were smaller and finer, and evenly distributed than that of LC-0. When the surface was modified by laser cladding, internal oxidation along grain boundary also decreased, but there was almost no difference between sample LC-1, 2, 3. The reason would be attributed to the melt pool convection flow and the evolved solidification structure during the laser cladding process.