Plan-view and cross-sectional characterization of thiourea-treated phosphorus-added steel surface

The galvanizing reaction is retarded when phosphorus-added steel is used as a substrate. We have found that both the galvanizing and the galvannealing reactions are highly promoted when thiourea solution is coated on the cold-rolled steel surface before the annealing process. Both a plan-view and cross-sectional characterization of thiourea-coated steel was performed using low-voltage scanning electron microscopy and a focused ion beam. A fine grain structure is formed in the surface region of ∼1 μm in thickness. In this region, (Mn, Fe)S particles are formed by the reaction between sulphur from thiourea and manganese from steel. These (Mn, Fe)S particles have a pinning effect on the cold-rolled steel grains and greatly retard recrystallization when the steel is annealed. It is concluded that the promotion of the galvannealing reaction is due to the increased diffusion paths of zinc and iron materialized by the fine grain structure of the thiourea-coated steel.