An Improved Hot Dip Coating Process

• Thin sheet of AHSS grades for automobile and structural applications are being protected by the corrosion resistant hot dip galvanizing coating. However, the surface selective segregation and oxidation tendency of AHSS grades makes difficult to obtain the good quality of galvanizing coating in continuous galvanizing line (CGL).
• The present approaches have their own merits and demerits. Therefore, there is need to develop an alternate cost effective technique to mitigate wettability issue of steel surface in galvanizing bath.
• The improved process of hot dip coating with the application of prior sol-gel iron based coating on the steel surface is industrially convenient and economically feasible process, alternative cost effective solution to the uses of high current electrodeposition pre-layer and oxidation reduction method

The present invention relates to steel industrial sectors of continuous hot dip galvanizing coating line for the cold rolled steel sheets of various grades being used for automobile and structural applications and protected by the corrosion resistant hot dip galvanizing coating.

• The improved process of hot dip coating with the application of prior sol-gel iron based coating (200 nm-800 nm) on the steel surface is developed with optimized process parameters and compatible to CGL line.
• Applicable to all grades of steel sheet (IFHS, DP 590, DP 780, DP 980)
• Applicable to all baths of hot dip coatings (GI, GA, ZAM, Galfan, Galvalume, Al-Si)
• No need to vary the annealing atmospheric conditions (Gas Mixture and Dew Point)
• The thin metallic Fe based layer on the steel surface with prior sol-gel coating and its subsequent annealing-reduction prevents the surface selective oxidation of alloying elements content into the steels.
• The thin metallic pre-layer on the steel surface, just before dipping into molten metal bath of hot dip coating line, provides good wettability and improve the galvanizability of steel surface.

The galvanizability of AHSS steel sheet has been improved with ease and compatible commercial operations in single dip process of sol gel pre layer application.